Smart, yes, but he's a bit of a fool on the quiet (3)
I believe the answer is:
fop
'smart yes but he's a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'bit of a fool on the quiet' is the wordplay.
'bit of' indicates taking the first letters.
'the quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
The initial letters of 'fool on' is 'fo'.
'fo'+'p'='FOP'
Can you help me to learn more?
(Other definitions for fop that I've seen before include "A dandy like Beau Brummel" , "he enjoys the finest dressing" , "Man excessively concerned with fashion" , "A dandy, overdressed man" , "A man much concerned with his appearance, a dandy" .)