This could replace spinach or pine (6)
I believe the answer is:
orache
'this could replace spinach' is the definition.
(vegetable which may be substituted for spinach)
'or pine' is the wordplay.
'pine' becomes 'ache' (synonyms).
'or'+'ache'='ORACHE'
(Other definitions for orache that I've seen before include "spinach substitute" , "Edible spinach-like plant of the genus Atriplex that thrives in arid areas" , "vegetable" .)