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, adjectives and past participle verbs can sometimes define each other.
'back us again' is the wordplay.
'back' says the letters should be written backwards.
'again' becomes 're'.
'us' written backwards gives 'su'.
'su'+'re'='SURE'
'to' is the 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" .)