Stray GI back in outfit (7)
I believe the answer is:
digress
'stray' is the definition.
(I know that stray can be written as digress)
'gi back in outfit' is the wordplay.
'back' says the letters should be written in reverse.
'in' is an insertion indicator.
'outfit' becomes 'dress' (I've seen this in other clues).
'gi' written backwards gives 'ig'.
'ig' inserted inside 'dress' is 'DIGRESS'.
(Other definitions for digress that I've seen before include "Go off topic" , "Depart from the main subject" , "Go off at a tangent" , "Get off the point" , "Leave the main subject" .)