Inspect stallion approaching filly's rear (5)
I believe the answer is:
study
'inspect' is the definition.
(thesaurus)
'stallion approaching filly's rear' is the wordplay.
'stallion' becomes 'stud' (stud is a kind of stallion).
'approaching' is a charade indicator (letters next to each other).
'rear' suggests the final letters (the back of the word).
The final letter of 'filly' is 'y'.
'stud'+'y'='STUDY'
(Other definitions for study that I've seen before include "Learn; den" , "Read up - academic work" , "Analysis" , "See 11" , "Small office" .)