Bound to back us again (4)
I believe the answer is:
sure
'bound to' is the definition.
(synonyms)
'back us again' is the wordplay.
'back' is a reversal indicator.
'again' becomes 're' (prefix meaning 'again').
'us' in reverse letter order is 'su'.
'su'+'re'='SURE'
(Other definitions for sure that I've seen before include "Confident sort of ruse" , "Definite" , "stable" , "Undoubtedly true" , "Safe" .)