Regulation I ignored in making guns (8)
I believe the answer is:
ordnance
'guns' is the definition.
(ordnance can mean guns or artillery)
'regulation i ignored in' is the wordplay.
'regulation' becomes 'ordinance' (synonyms).
'ignored in' suggests deleting specific letters.
'ordinance' with 'i' taken away is 'ORDNANCE'.
'making' is the link.
(Other definitions for ordnance that I've seen before include "weaponry" , "Arsenal's big guns" , "Car on end (anag) - artillery" , "weapons" , "Cannon; munitions" .)