A grain seed (5)
I believe the answer is:
acorn
'seed' 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'
(Other definitions for acorn that I've seen before include "Fruit of the oak tree" , "From which a mighty oak may grow?" , "From which a "tall oak" may grow" , "From which a tall tree may grow" , "Model in stone" .)