Tree potentially has a grain (5)
I believe the answer is:
acorn
'tree potentially' is the definition.
(I've seen this before)
'a grain' is the wordplay.
'grain' becomes 'corn' (corn is a kind of grain).
'a'+'corn'='ACORN'
'has' acts as a link.
(Other definitions for acorn that I've seen before include "from which something bigger may develop" , "form of seeding" , "oak's fruit" , "Fruit of the oak tree" , "From which a tall tree may grow" .)