I was looking at Collins' definition of double-cross:
If someone you trust double-crosses you, they do something which harms you instead of doing something they had promised to do.
So we have double-crosses and harms in present simple, but then the last choice of tense and aspect is had promised in past perfective. Can someone explain to me why it cannot be present perfective? What is the point of reference in time for this past perfective?
One use of the past perfect, according to Cambridge dictionaries, is where you're talking about a changed state, something that was the case but is no longer the case. For instance where someone promised something, then broke that promise. This can happen even when you're using the present tense.
They give a couple of examples:
A: Are you going anywhere today?
B: I had planned to go to the beach but look at the rain!
They note that here "had is stressed; the meaning is ‘I have now changed my mind’". It emphasises that B no longer wants to go to the beach, unlike "I planned to go to the beach but look at the rain!" In the latter case they might still go to the beach even though it's raining.
I’m very happy working as an engineer but I had wanted to be an actor when I was younger.
In the latter case the speaker is emphasising that they no longer want to be an actor, while "I’m very happy working as an engineer but I wanted to be an actor when I was younger." might suggest that they still want to be an actor even if they aren't one now.
It wouldn't be wrong to use the present perfect "...something they have promised to do", but "had promised" emphasised something that was done but is no longer the case: the promise is broken.
they had promised to do. = [that] they had promised to do.
This is a subordinate, relative clause that modifies "something".
The tense of subordinate clauses are not (in general) affected by the tense of the main clause.
Why choose the past perfect?
The past perfect is used to describe context and background, i.e. events that finished prior to the main event. The main event is "double crossing you".
Your example is a correct use of the past perfect.
Compare
I had dug the garden, I had taken the dog for a walk (things that had finished before the main event and give context) and then the visitors arrived (The main event).
