Slip up in the gun room (6)
I believe the answer is:
garret
'room' is the definition.
(I've seen this before)
'slip up in the gun' is the wordplay.
'slip' becomes 'err' (to err is to make an error).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'in' is an insertion indicator.
'the gun' becomes 'gat' (US informal term for a gun).
'err' reversed gives 'rre'.
'rre' placed within 'gat' is 'GARRET'.
(Other definitions for garret that I've seen before include "Upper room" , "Attic, room at the top" , "Room under the roof" , "Room at the top where artists starved" , "Attic space" .)