It's unchanged when returned to me (4)
I believe the answer is:
same
'it's unchanged' is the definition.
(similar in meaning)
'when returned to me' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'returned' is a reversal indicator.
'as' in reverse letter order is 'sa'.
'sa'+'me'='SAME'
(Other definitions for same that I've seen before include "Aforesaid" , "Identical, unchanged" , "If - be the food of love, play on (Twelfth Night)" , "uniform" , "Identical, not different" .)