Completed? In return, we nod (4)
I believe the answer is:
done
'completed?' is the definition.
(something that's been completed has been done)
'return we nod' is the wordplay.
'return' is a reversal indicator.
'we' becomes 'e' (I can't justify this - if you can you should believe this answer much more).
'e'+'nod'='enod'
'enod' written backwards gives 'DONE'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for done that I've seen before include "This settled" , "Achieved" , "Concluded (and dusted?)" , "Thoroughly cooked; accomplished" , "This sort of thing is socially acceptable" .)