Sister entertains at group's pleasure (12)
I believe the answer is:
satisfaction
'pleasure' is the definition.
(thesaurus)
'sister entertains at group's' is the wordplay.
'sister' becomes 'sis' ('sis' can be a synonym of 'sister').
'entertains' means one lot of letters goes inside another.
'group' becomes 'faction' (faction is a kind of group**).
'sis' going around 'at' is 'satis'.
'satis'+'faction'='SATISFACTION'
(Other definitions for satisfaction that I've seen before include "Fulfilment; compensation" , "Gratification" , "I can't get no - (Rolling Stones)" , "Pleasure" , "Contentment; reparation" .)