A grain, or fruit (5)
I believe the answer is:
acorn
'fruit' is the definition.
(acorn is a kind of fruit)
'a grain' is the wordplay.
'grain' becomes 'corn' (corn is a kind of grain).
'a'+'corn'='ACORN'
'or' is the link.
(Other definitions for acorn that I've seen before include "Potential oak" , "from which something bigger may develop" , "Fruit of the oak tree" , "From which a tall tree may grow" , "old computer?" .)