Contentment is found in assembled clique (12)
I believe the answer is:
satisfaction
'contentment' is the definition.
(satisfaction is a kind of contentment)
'is found in assembled clique' is the wordplay.
'found in' means one lot of letters goes inside another.
'assembled' becomes 'sat' (I've seen this in another clue).
'clique' becomes 'faction' (faction is a kind of clique).
'sat'+'faction'='satfaction'
'is' put into 'satfaction' is 'SATISFACTION'.
(Other definitions for satisfaction that I've seen before include "Contentment; reparation" , "Pleasure" , "I can't get no - (Rolling Stones)" , "Gratification" , "Fulfilment; compensation" .)