Twins have to gain (3)
I believe the answer is:
win
'gain' is the definition.
(win is a kind of gain)
'twins have' is the wordplay.
'have' indicates the answer is hidden within the clue.
'WIN' is hidden within 'twins'.
'to' is the link.
(Other definitions for win that I've seen before include "Trumph" , "Come first" , "Achieve victory" , "Be victorious or succeed" , "Finish in first place" .)