Guns in backstage American usage (4)

'guns' is the definition.
(gat is a kind of gun)

'backstage american usage' is the wordplay.
I cannot really understand how this works, but
'american' could be 'a' (common abbreviation - e.g. in organisation names) and 'a' is located in the answer.
The remaining letters 'gts' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

'in' is the link.

Can you help me to learn more?