It's in an army corps, this ceremony (4)
I believe the answer is:
rite
'this ceremony' is the definition.
(rite is a kind of ceremony)
'it's in an army corps' is the wordplay.
'it's' becomes 'it' (I've seen this in other clues).
'in' indicates putting letters inside.
'an army corps' becomes 're' (I've seen this in other clues).
'it' inserted within 're' is 'RITE'.
(Other definitions for rite that I've seen before include "Established religious ceremony" , "Tries awkwardly to perform rituals" , "form of service" , "Uncomplaining person" , "Mystery - ceremony" .)