Bighead, past his prime, yet courageous (4)
I believe the answer is:
bold
'courageous' is the definition.
(associated in meaning)
'bighead past' is the wordplay.
'bighead' becomes 'b' (the head letter of 'big').
'past' becomes 'old' (associated in meaning).
'b'+'old'='BOLD'
'his prime yet' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for bold that I've seen before include "Intrepid" , "Confident, daring" , "Excessively forward" , "Brash" , "No chicken" .)