Practice of firing in large plant losing a year (7)
I believe the answer is:
gunnery
'practice of firing' is the definition.
(firing guns)
'large plant losing a year' is the wordplay.
'large plant' becomes 'gunnera' (type of plant).
'losing' suggests deleting specific letters.
'year' becomes 'y'.
'gunnera' with 'a' removed is 'gunner'.
'gunner'+'y'='GUNNERY'
'in' acts as a link.
(Other definitions for gunnery that I've seen before include "Artilleryman's skill" , "Use of heavy artillery" , "(Use of) heavy weapons systems" .)