Fully expected to back us again (4)
I believe the answer is:
sure
'fully expected' is the definition.
The answer and definition are different parts of speech. However, past participle verbs and adjectives occasionally mean the same thing.
'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'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for sure that I've seen before include "Feeling no doubt" , "Certain to be a ruse" , "The old sewer" , "Convinced" , "Undoubtedly true" .)