Finery originally introduced in army corps celebration (7)
I believe the answer is:
regalia
'finery' is the definition.
('regalia' can be a synonym of 'finery')
'originally introduced in army corps celebration' is the wordplay.
'originally' says to take the initial letters.
'in' is an insertion indicator.
'army corps' becomes 're' (I've seen this in other clues).
'celebration' becomes 'gala' (gala is a kind of celebration**).
The first letter of 'introduced' is 'i'.
're'+'gala'='regala'
'i' placed inside 'regala' is 'REGALIA'.
(Other definitions for regalia that I've seen before include "Finery" , "Royal items" , "Royal paraphernalia" , "Crown jewels" , "Crown and associated pieces" .)