Half were certain to come back (7)
I believe the answer is:
rebound
'come back' is the definition.
(I've seen this in another clue)
'half were certain' is the wordplay.
'half' suggests halving the letters.
'certain' becomes 'bound' (as in the phrase 'it's bound to be').
'were' halved is 're'.
're'+'bound'='REBOUND'
'to' acts as a link.
(Other definitions for rebound that I've seen before include "Backfire" , "misfire" , "Spring back, bounce" , "be resilient" , "Come straight back" .)