Is there a single word denoting "field of interest", or perhaps "field of expertise"?
E.g.: I'm looking for a mentor in my new field of interest.
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Sign up to join this communityIf you have some objection to saying "field of interest," you might say "discipline." A "discipline" is an area of study. I wouldn't use "field of expertise" if you don't yet have expertise in that field, which it sounds like you don't since you refer to it as "new" and are seeking mentorship.
Another word you might use is "subject." A "subject" is also an area of study but can have a less-broad connotation than "discipline" does because of being more closely associated with coursework, but that isn't necessarily the case.
Consider the noun métier. Though, I must warn you that this one by no means is an everyday word:
noun
a field of work; occupation, trade, or profession.
a field of work or other activity in which one has special ability or training; forte.
speciality or specialty:
1] a special interest or skill; a special subject of study, line of work, skill, or the like on which one concentrates. TFD