Organ player ultimately wrong to be upset (5)
I believe the answer is:
liver
'organ' is the definition.
(liver is a kind of organ)
'player ultimately wrong to be upset' is the wordplay.
'ultimately' suggests the final letters.
'wrong' becomes 'evil' (associated in meaning).
'to be upset' is a reversal indicator (I've seen 'being upset' mean this).
The final letter of 'player' is 'r'.
'r'+'evil'='revil'
'revil' in reverse letter order is 'LIVER'.
(Other definitions for liver that I've seen before include "Organ damaged by excess alcohol" , "Bodily organ" , "Detoxifying organ" , "Glandular organ of the stomach" , "Organ; reddish-brown" .)