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' indicates the answer is hidden within the clue (I've seen 'a bit' mean this (the answer is formed by a bit of 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" .)