Supporter upset New York players (4)
I believe the answer is:
stem
'supporter' is the definition.
(I've seen this before)
'upset new york players' is the wordplay.
'upset' says the letters should be written backwards.
'new york players' becomes 'mets'.
'mets' reversed gives 'STEM'.
(Other definitions for stem that I've seen before include "Hold back - stock - check" , "Staunch (flow)" , "Restrict - part of a plant" , "supporter of flag, for example" , "Thwart; support" .)