Hip worker inhaling class grass (9)
I believe the answer is:
informant
'grass' is the definition.
(I know that grass can be written as informant)
'hip worker inhaling class' is the wordplay.
'hip' becomes 'in' (both can mean fashionable).
'worker' becomes 'ant' (worker is a type of ant).
'inhaling' is an insertion indicator.
'class' becomes 'form class' (synonyms).
'in'+'ant'='inant'
'inant' placed around 'form' is 'INFORMANT'.
(Other definitions for informant that I've seen before include "source of intelligence" , "Grass" , "Imparter of knowledge" , "Rat" , "Sneak" .)