Sappers have an opportunity to go back (6)
I believe the answer is:
return
'go back' is the definition.
(returning is a kind of going back)
'sappers have an opportunity' is the wordplay.
'sappers' becomes 'RE' (nickname for the Royal Engineers).
'have an opportunity' becomes 'turn' (I can't justify this - if you can you should believe this answer much more).
're'+'turn'='RETURN'
'to' is the link.
Can you help me to learn more?
(Other definitions for return that I've seen before include "reply" , "Give or come back" , "Homecoming" , "Come back; profit" , "answer" .)