Minute, hot insect under stone (7)
I believe the answer is:
instant
'minute' is the definition.
(synonyms)
'hot insect under stone' is the wordplay.
'hot' becomes 'in' (both can mean fashionable).
'insect' becomes 'ant' (ant is a kind of insect).
'under' is a charade indicator (letters next to each other).
'stone' becomes 'st'.
'ant' after 'st' is 'stant'.
'in'+'stant'='INSTANT'
(Other definitions for instant that I've seen before include "Trice" , "Without delay - short time" , "Occurring immediately, like some fast food" , "No need to cook" , "Tick" .)