Cow's comment about races becomes slogan (5)
I believe the answer is:
motto
'slogan' is the definition.
(I know that slogan can be written as motto)
'cow's comment about races' is the wordplay.
'cow's comment' becomes 'moo' (I've seen this in another clue).
'about' is an insertion indicator.
'races' becomes 'tt' (Isle of Man TT).
'moo' enclosing 'tt' is 'MOTTO'.
'becomes' acts as a link.
(Other definitions for motto that I've seen before include "Brief guiding rule" , "Christmas cracker item" , "Saying; slogan" , "Words often found on coat of arms" , "statement of belief" .)