Novice’s first use of firearms not good in religious establishment (7)
I believe the answer is:
nunnery
'religious establishment' is the definition.
(I've seen this before)
'novice's first use of firearms not good' is the wordplay.
'novice's first' becomes 'n' (1st letter of 'novice').
'use of firearms' becomes 'gunnery' (using guns).
'not' indicates named letters should be taken away.
'good' becomes 'g' (abbreviation).
'gunnery' with 'g' taken away is 'unnery'.
'n'+'unnery'='NUNNERY'
'in' acts as a link.
(Other definitions for nunnery that I've seen before include "Female religious house" , "Convent" , "sisters' place" , "Hamlet to Ophelia: 'Get thee to a . . . . . . .'" , "Religious establishment" .)