Saw a knight after travail (6)
I believe the answer is:
slogan
'saw' is the definition.
(saw can mean a saying or slogan)
'a knight after travail' is the wordplay.
'knight' becomes 'n' (chess abbreviation).
'after' means one lot of letters go next to another.
'travail' becomes 'slog' (both can mean a strenuous task or struggle).
'a'+'n'='an'
'an' put after 'slog' is 'SLOGAN'.
(Other definitions for slogan that I've seen before include "Short catchy phrase used in advertising" , "Catchy phrase in advertisement etc" , "Word or phrase adopted as motto" , "Memorable advertising phrase" , "Shibboleth" .)