Starmer's first aim for Labour (6)
I believe the answer is:
strain
'for labour' is the definition.
(both can mean to work hard)
'starmer's first aim' is the wordplay.
'starmer's first' becomes 's' (1st letter of 'starmer').
'aim' becomes 'train' (synonyms).
's'+'train'='STRAIN'
(Other definitions for strain that I've seen before include "Overwork - melody" , "Exert oneself to the utmost" , "In arts (anag)" , "Tension - stock" , "Force - melody" .)