His party has to fight (4)
I believe the answer is:
spar
'fight' is the definition.
(I know this)
'his party has' is the wordplay.
'has' indicates a hidden word.
'SPAR' can be found hidden inside 'his party'.
'to' acts as a link.
(Other definitions for spar that I've seen before include "Rafter, boom" , "Translucent mineral" , "Box (lightly) - fight (verbally)" , "Act as boxer" , "microcrystalline mineral?" .)