Exchange hands when returning (4)
I believe the answer is:
swap
'exchange' is the definition.
(I know that swap is a type of exchange)
'hands when returning' is the wordplay.
'hands' becomes 'paws' ('paw' can be a synonym of 'hand').
'when returning' shows that the letters should be reversed in order.
'paws' in reverse letter order is 'SWAP'.
(Other definitions for swap that I've seen before include "Exchange goods" , "trade" , "Exchange, substitute" , "exchanged gift" , "Fifty-fifty odds" .)