Soldiers certain to come back (7)
I believe the answer is:
rebound
'come back' is the definition.
(I've seen this before)
'soldiers certain' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'certain' becomes 'bound' (as in the phrase 'it's bound to be').
're'+'bound'='REBOUND'
'to' is the link.
(Other definitions for rebound that I've seen before include "Backfire" , "Echo back" , "Produce an own goal, perhaps" , "Spring back -- unrobed (anag)" , "Spring back on impact" .)