Fit in with conservative class (7)
I believe the answer is:
conform
'fit in' is the definition.
(thesaurus)
'with conservative class' is the wordplay.
'with' means one lot of letters go next to another.
'conservative' becomes 'con' (I've seen this in other clues).
'class' becomes 'form' (form class is a kind of class**).
'con' next to 'form' is 'CONFORM'.
(Other definitions for conform that I've seen before include "Toe the line" , "Comply with accepted rules" , "Fall into line" , "Follow the crowd" , "Run with the pack" .)