Holding line, it rarely disturbed gunners (9)
I believe the answer is:
artillery
'gunners' is the definition.
(I've seen this before)
'holding line it rarely disturbed' is the wordplay.
'holding' means one lot of letters goes inside another.
'line' becomes 'l'.
'disturbed' is an anagram indicator.
'it'+'rarely'='itrarely'
'itrarely' is an anagram of 'artilery'.
'l' going into 'artilery' is 'ARTILLERY'.
(Other definitions for artillery that I've seen before include "Heavy weapons" , "Large guns for army in land war" , "group of soldiers" , "Gunners" , "Ordnance" .)