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' says the letters should be written in reverse.
'again' becomes 're' (prefix meaning 'again').
'us' backwards is 'su'.
'su'+'re'='SURE'
(Other definitions for sure that I've seen before include "Confident sort of ruse" , "Definite" , "stable" , "Undoubtedly true" , "Safe" .)