Tree coming up in season possibly producing protein-rich food (4,5)
I believe the answer is:
soya beans
'protein-rich food' is the definition.
'tree coming up in season possibly' is the wordplay.
'tree' becomes 'bay' (bay tree is a kind of tree).
'coming up' is a reversal indicator.
'in' indicates putting letters inside.
'possibly' indicates anagramming the letters.
'bay' written backwards gives 'yab'.
'season' with letters rearranged gives 'soeans'.
'yab' going within 'soeans' is 'SOYA BEANS'.
'producing' is the link.
(Other definitions for soya beans that I've seen before include "Asian pulses used as a source of oil" , "Vegetable source of protein" .)