Support offered by Serena's sister (6)
I believe the answer is:
assist
'support' is the definition.
(assisting is a kind of supporting)
'offered by serena's sister' is the wordplay.
'offered by' indicates the answer is hidden within the clue.
'ASSIST' is hidden within 'serenas sister'.
(Other definitions for assist that I've seen before include "providing help?" , "Provide aid, help out" , "benefit" , "Help to make" , "Give a hand to" .)