Rough-spoken insolence that groom must polish up? (5,5)
I believe the answer is:
horse brass
'groom must polish up?' is the definition.
I can't judge whether this defines the answer.
'rough-spoken insolence' is the wordplay.
'rough' becomes 'hoarse' (I've seen this before).
'spoken' indicates a 'sounds like' (homophone) clue.
'insolence' becomes 'brass' (I can't explain this - if you can you should give a lot more credence to this answer).
'hoarse' is a homophone of 'horse'.
'horse'+'brass'='HORSE BRASS'
'that' is the link.
Can you help me to learn more?
(Another definition for horse brass that I've seen is " Decorative ornament originally attached to a harness".)