For this operation, I had to join the artillery (4)
I believe the answer is:
raid
'for this operation' is the definition.
(raid is a kind of operation)
'i had to join the artillery' is the wordplay.
'i had' becomes 'i'd' (contraction).
'to join' means one lot of letters go next to another.
'the artillery' becomes 'RA' (Royal Artillery).
'id' put after 'ra' is 'RAID'.
(Other definitions for raid that I've seen before include "Foray, sudden attack" , "Sudden and surprise attack" , "Inroad" , "Go on the attack" , "Rifle" .)