Function of Starmer? Primarily to go after Conservative! (3)
I believe the answer is:
cos
'function' is the definition.
(mathematical function)
'of starmer? primarily to go after conservative' is the wordplay.
'primarily' says to take the initial letters.
'to go after' is a charade indicator (letters next to each other).
'conservative' becomes 'C' (UK political abbreviation).
The initial letters of 'of starmer' is 'os'.
'os' put after 'c' is 'COS'.
(Other definitions for cos that I've seen before include "Chief of Staff" , "Greek island (alternative spelling): Trig abbrev!" , "Variety of lettuce" , "Island in Mediterranean" , "Senior officers" .)