Support posterior and sit awkwardly (6)
I believe the answer is:
assist
'support' is the definition.
(assisting is a kind of supporting)
'posterior and sit awkwardly' is the wordplay.
'posterior' becomes 'ass' ('ass' can be a synonym of 'posterior').
'and' is a charade indicator (letters next to each other).
'awkwardly' indicates an anagram.
'sit' with letters rearranged gives 'ist'.
'ass'+'ist'='ASSIST'
(Other definitions for assist that I've seen before include "Give a hand to" , "Serve" , "providing help?" , "Pass to scorer" , "Help, lend a hand" .)