Hit on an advertising catchphrase (6)
I believe the answer is:
slogan
'advertising catchphrase' is the definition.
(I've seen this before)
'hit on an' is the wordplay.
'hit' becomes 'slog' (slogging is a kind of hitting**).
'on' says to put letters next to each other.
'slog'+'an'='SLOGAN'
(Other definitions for slogan that I've seen before include "Identifying catchword" , "Memorable advertising phrase" , "Shibboleth" , "Marketing phrase" , "Battle cry - advertising phrase" .)