Where clothing's concerned, he won't take anything off (3)
I believe the answer is:
don
'where clothing's concerned he won't' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'take anything off' is the wordplay.
'take' becomes 'do' (eg taking or doing drugs).
'anything off' becomes 'n' (I can't explain this - if you can you should believe this answer much more).
'do'+'n'='DON'
Can you help me to learn more?
(Other definitions for don that I've seen before include "University lecturer" , "A mafia chief or Spanish lord" , "A mafia chief or Spanish grandee" , "River, academic" , "University teacher" .)