Ask about fashionable creature (5)
I believe the answer is:
being
'creature' is the definition.
(thesaurus)
'ask about fashionable' is the wordplay.
'ask' becomes 'beg' (begging is a kind of asking).
'about' is an insertion indicator.
'fashionable' becomes 'in' ('in' can be similar in meaning to 'fashionable').
'beg' placed around 'in' is 'BEING'.
(Other definitions for being that I've seen before include "Living thing" , "substance" , "Existence" , "Person" , "See 9" .)