Labour leader of Sunday Express? (6)
I believe the answer is:
strain
'labour' is the definition.
(both can mean to work hard)
'leader of sunday express?' is the wordplay.
'leader of' says to take the initial letters.
'express?' becomes 'train' (express is a kind of train).
The initial letter of 'sunday' is 's'.
's'+'train'='STRAIN'
(Other definitions for strain that I've seen before include "Injure (a muscle); distinct breed" , "Trace" , "Struggle" , "Sieve; breed" , "Pass through a colander" .)