Second man fitting in chief officer (7)
I believe the answer is:
captain
'chief officer' is the definition.
'captain' can be an answer for 'officer' (captain is a kind of officer). I am unsure of the 'chief' bit.
'second man fitting in' is the wordplay.
'second man' becomes 'cain' (biblical second man after Adam).
'fitting' becomes 'apt' (both mean appropriate).
'in' indicates putting letters inside.
'cain' placed around 'apt' is 'CAPTAIN'.
Can you help me to learn more?
(Other definitions for captain that I've seen before include "US policeman" , "Chief" , "Naval rank" , "Lead (a team)" , "Is Pat Cain in charge of the ship?" .)