Stray soldier back in outfit (7)
I believe the answer is:
digress
'stray' is the definition.
(synonyms)
'soldier back in outfit' is the wordplay.
'soldier' becomes 'GI' (GI can mean a US soldier).
'back' says the letters should be written backwards.
'in' indicates putting letters inside.
'outfit' becomes 'dress' (I've seen this in another clue).
'gi' written backwards gives 'ig'.
'ig' inserted within 'dress' is 'DIGRESS'.
(Other definitions for digress that I've seen before include "Leave the main subject" , "Ramble" , "Deviate from the main subject" , "Go off topic" , "Depart from the main subject" .)