. . . to return gold in another's RMB (4)
I believe the answer is:
yuan
'rmb' is the definition.
I can't tell whether this definition defines the answer.
'to return gold in another's' is the wordplay.
'to return' shows that the letters should be reversed in order.
'gold' becomes 'Au' (Au is the chemical symbol for gold).
'in' is an insertion indicator.
'another's' becomes 'ny' (I can't explain this - if you can you should believe this answer much more).
'au' placed within 'ny' is 'nauy'.
'nauy' backwards is 'YUAN'.
Can you help me to learn more?
(Other definitions for yuan that I've seen before include "money abroad" , "Chinese capital" , "Chinese money" , "Income in Shanghai" , "Oriental ready" .)