Advise on fashionable shape (6)
I believe the answer is:
inform
'advise' is the definition.
(I know that advise is a more specific form of inform)
'on fashionable shape' is the wordplay.
'on' is a charade indicator (letters next to each other).
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'shape' becomes 'form' (synonyms).
'in' put next to 'form' is 'INFORM'.
(Other definitions for inform that I've seen before include "Tell - shop" , "Grass - doing well" , "Spill the beans" , "Advise, acquaint with" , "Enlighten" .)