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' means to take half the letters.
'certain' becomes 'bound' (as in the phrase 'it's bound to be').
'were' with half the letters taken is 're'.
're'+'bound'='REBOUND'
'to' is the link.
(Other definitions for rebound that I've seen before include "Backfire" , "misfire" , "Spring back, bounce" , "be resilient" , "Come straight back" .)