Clean up old man with grim haircut (9)
I believe the answer is:
pompadour
'haircut' is the definition.
The definition and answer can be both body parts as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'clean up old man with grim' is the wordplay.
'clean' becomes 'mop' (mopping is a kind of cleaning).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'old man' becomes 'pa' (both can mean someone's father).
'with' is a charade indicator (letters next to each other).
'grim' becomes 'dour' (synonyms).
'mop' in reverse letter order is 'pom'.
'pom'+'pa'+'dour'='POMPADOUR'
Can you help me to learn more?
(Other definitions for pompadour that I've seen before include "18th century women's hairstyle" , "18th century hairstyle employing a pad at the front to add bulk" , "Lady" , "High Barnet" , "Extravagant quiff" .)