Certainly you can depend on us being back (6)
I believe the answer is:
surely
'certainly' is the definition.
(I know that certainly can be written as surely)
'depend on us being back' is the wordplay.
'depend' becomes 'rely' (to depend on someone is to rely on them).
'on' says to put letters next to each other.
'being back' says the letters should be written in reverse.
'us' in reverse letter order is 'su'.
'rely' after 'su' is 'SURELY'.
'you can' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for surely that I've seen before include "it would seem ironic?" , "Undoubtedly" , "Without doubt, certainly" , "Confidently" , "with firmness" .)