Medium upset couple (4)
I believe the answer is:
mate
'couple' is the definition.
(I know that couple can be written as mate)
'medium upset' is the wordplay.
'medium' becomes 'm' (as in UK clothing sizes).
'upset' becomes 'ate' (to eat up is worry or upset).
'm'+'ate'='MATE'
(Other definitions for mate that I've seen before include "Marry - friend" , "Ship's officer or marital partner" , "Finish game" , "Spouse or informal pal (4)" , "Freind, companion" .)