Royal Engineers go, then come back (6)
I believe the answer is:
return
'come back' is the definition.
(synonyms)
'royal engineers go' is the wordplay.
'royal engineers' becomes 'RE' (abbreviation).
'go' becomes 'turn' (as in one's go or turn in a game).
're'+'turn'='RETURN'
'then' acts as a link.
(Other definitions for return that I've seen before include "reply" , "Send/come back" , "hit back?" , "Come back; profit" , "Give or come back" .)