Big guns with one in command (9)
I believe the answer is:
ordinance
'command' is the definition.
(I've seen this before)
'big guns with one in' is the wordplay.
I cannot quite see how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is found within the answer.
'in' is within the answer.
This explanation may well be incorrect...
(Other definitions for ordinance that I've seen before include "practice" , "Command" , "law" , "Edict, order" , "Bill" .)