Certainly depend on you and me returning (6)
I believe the answer is:
surely
'certainly' is the definition.
(I know that certainly can be written as surely)
'depend on you and me returning' is the wordplay.
'depend' becomes 'rely' (to depend on someone is to rely on them).
'on' is a charade indicator (letters next to each other).
'you and me' becomes 'us'.
'returning' says the letters should be written backwards.
'us' back-to-front is 'su'.
'rely' put after 'su' is 'SURELY'.
(Other definitions for surely that I've seen before include "Without doubt, certainly" , "Confidently" , "With certainty" , "Definitely or positively" , "it would seem ironic?" .)