Fitting in again with leader of military ceremony (7)
I believe the answer is:
baptism
'ceremony' is the definition.
(baptism is a kind of ceremony)
'fitting in again with leader of military' is the wordplay.
'fitting' becomes 'apt' (I've seen this before).
'in' is an insertion indicator.
'again' becomes 'bis' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'leader of military' becomes 'm' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'apt' placed into 'bis' is 'baptis'.
'baptis'+'m'='BAPTISM'
(Other definitions for baptism that I've seen before include "Initiation; christening" , "Christian sacrament that symbolises admission to the church" , "Initiation ceremony" , "Christening ceremony" , "Christian sacrament signifying rebirth" .)