Classical terminology is of very little use in describing English poetry. Looking for ‘feet’ in an English line is like trying to analyze a Louis Armtrong or Joe Morello solo by categorizing the 28 patterns of measure employed: it’s an academic exercise which has very little to do with what’s actually going on. What’s important is the bar lines.
The fundamental rhythm of English poetry, from Cædmon on down, is the four-stress line — most naively in what we call ‘common meter’:
Praise |God from |whom all |blessings |flow
Praise |Him all |creatures |here be|low
Ballad meter, the ‘fourteener’, is two four-stress lines with a rest at the end of the second:
The |day doth |daw, the |cock doth |craw, the |channering |worm doth |chide |
Note that tri-syllable on ‘channering’. You can call it a ‘dactyl’, but it’s not; it’s a triplet. Housman was one of the great Latin scholars of his generation, but it’s impossible to imagine that he set himself to build with paeons:
‘In the |back back |garden Thoma|sina, |
Did you |recently vo|ciferate a |squeal?’ |
‘Oh, I |trod upon an |amphis|baena |
And it|bit me on the |toe and on the |heel.’ |
The ‘rule’ is that you can pick up any stress in the line (including the first) with any quantity of unstressed syllables, or none, so long as you maintain the four-stress pulse. This is equally true of the classic pentameter line; just look at the most famous lines of our most famous pentameter poets:
|Whan that |aprill with his |shourés |sooté
The |droghte of |March hath |percéd to the |rooté ...
To |be, or |not to be, |that is the |question:
|Whether ’tis |nobler in the |mind to |suffer ...
Of |man’s |first diso|bedience and the |fruit
Of that for|bidden |tree whose |mortal |taste ...
A |little |learning is a |dangerous |thing;
Drink |deep, or |taste not the Pi|erian |spring ...
The great power of the pentameter line lies in the tension between the nominal five-foot meter and the underlying four-stress pulse. (There’s a very good treatment of this in the ‘Fourth Essay’ of Northrop Frye’s Anatomy of Criticism.) Long-form unrhymed poets like Shakespeare and Milton have many solutions, but in shorter forms the most common resolution is to de-stress one foot, to throw its stress onto the backbeat.
Here’s what Clare is up to; I've put the de-stressed syllables in italics. Note the very regular shift of the backbeat every third line from the 2-3 opening to the 3-4; the hammered bare downbeat after the only enjambment, which launches the climactic stanza; the painful isolation of ‘vast’ and ‘strange’ at the emotional high point; and the reservation of the 1-2 backbeat until almost the end, where it retards lines 17 and 18 to set up the very open final line.