Copper, British — he’s young (3)
I believe the answer is:
cub
'he's young' is the definition.
(cub can mean a young person or animal)
'copper british' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'cu'+'b'='CUB'
(Other definitions for cub that I've seen before include "'Baby bear, say (3)'" , "Young bear or lion" , "Young of certain animals" , "cheeky youngster" , "Baby animal, bear say" .)