Leader's fitting in the third person (7)
I believe the answer is:
captain
'leader's' is the definition.
(captain is a kind of leader)
'fitting in the third person' is the wordplay.
'fitting' becomes 'apt' (I've seen this before).
'in' means one lot of letters goes inside another.
'the third person' becomes 'Cain' (biblical third person after Adam and Eve).
'apt' put inside 'cain' is 'CAPTAIN'.
(Other definitions for captain that I've seen before include "Is Pat Cain in charge of the ship?" , "American police officer" , "Naval commander" , "Naval rank" , "Skipper" .)