Split variable in function (5)
I believe the answer is:
party
'function' is the definition.
(I've seen this before)
'split variable' is the wordplay.
'split' becomes 'part' ('part' can be a synonym of 'split').
'variable' becomes 'y' (I've seen this in other clues).
'part'+'y'='PARTY'
'in' acts as a link.
(Other definitions for party that I've seen before include "MP's faction" , "Crew" , "eg Labour, Conservative" , "push the boat out" , "Political organisation" .)