Sound a thick-headed person (4)
I believe the answer is:
tone
'sound' is the definition.
(I know that tone is a type of sound)
'a thick-headed person' is the wordplay.
'a thick-headed' becomes 't' (this might be a standard abbreviation which I don't know about).
'person' becomes 'one' ('one' can mean an unspecified person).
't'+'one'='TONE'
Can you help me to learn more?
(Other definitions for tone that I've seen before include "Pitch of a voice" , "Colour quality; note" , "Pitch (sound)" , "Musical note; general character" , "accent" .)