"Labour with vigour" - a new catchphrase (6)
I believe the answer is:
slogan
'catchphrase' is the definition.
(I've seen this before)
'labour with vigour a new' is the wordplay.
'labour with vigour' becomes 'slog'.
'a new' becomes 'an' (I've seen this in other clues).
'slog'+'an'='SLOGAN'
(Other definitions for slogan that I've seen before include "Short catchy phrase used in advertising" , "Advertising catchphrase" , "Tag line" , "Ad. catchphrase" , "Memorable advertising phrase" .)