Stray soldier turned up in uniform (7)
I believe the answer is:
digress
'stray' is the definition.
(I know that stray can be written as digress)
'soldier turned up in uniform' is the wordplay.
'soldier' becomes 'GI' (GI can mean a US soldier).
'turned up' says the letters should be written in reverse.
'in' means one lot of letters goes inside another.
'uniform' becomes 'dress' (dress uniform is a kind of uniform).
'gi' reversed gives 'ig'.
'ig' placed within 'dress' is 'DIGRESS'.
(Other definitions for digress that I've seen before include "Go off at a tangent" , "Wander from subject" , "Be sidetracked" , "Go off the point" , "Ramble" .)