Study a pose taken up aboard plane, perhaps (8)
I believe the answer is:
treatise
'study' is the definition.
(thesaurus)
'a pose taken up aboard plane perhaps' is the wordplay.
'pose' becomes 'sit' (sitting is a kind of posing**).
'taken up' says the letters should be written in reverse.
'aboard' means one lot of letters goes inside another.
'plane perhaps' becomes 'tree' (I've seen this in other clues).
'sit' reversed gives 'tis'.
'a'+'tis'='atis'
'atis' inserted within 'tree' is 'TREATISE'.
(Other definitions for treatise that I've seen before include "Iterates in dissertation" , "theory" , "Formal piece of writing on a particular subject" , "A written work about treaties" , "Academic composition" .)