He's a bit of a lunatic! (3)
I believe the answer is:
nat
'he's' is the definition.
'a bit of a lunatic' is the wordplay.
'a bit of' says the answer is hidden in the clue.
'NAT' is hidden in the letters of 'lunatic'.
(Other definitions for nat that I've seen before include "Politician" , "A nationalist" , "Supporter of the SNP (abbr)" , "King Cole" .)