Surprisingly won seat, Labour not being involved (2,5)
I believe the answer is:
no sweat
'labour not being involved' is the definition.
('no sweat' means without work or difficulty)
'surprisingly won seat' is the wordplay.
'surprisingly' indicates an anagram.
'won'+'seat'='wonseat'
'wonseat' anagrammed gives 'NO SWEAT'.
(Other definitions for no sweat that I've seen before include "East won (anag)" , "Not a problem! (informal)" , "That won't be a problem" , "OK" , "There shouldn't be any objection" .)