Slogan has low remark about races (5)
I believe the answer is:
motto
'slogan' is the definition.
(synonyms)
'low remark about races' is the wordplay.
'low remark' becomes 'moo' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' means one lot of letters goes inside another.
'races' becomes 'tt' (Isle of Man Tourist Trophy races).
'moo' placed around 'tt' is 'MOTTO'.
'has' acts as a link.
Can you help me to learn more?
(Other definitions for motto that I've seen before include "Phrase on, eg, family coat of arms" , "Short sentence adopted as rule of conduct" , "Slogan encapsulating self-image" , "Phrase in a Christmas cracker" , "Shibboleth" .)