A grain or a fruit (5)
I believe the answer is:
acorn
'a grain' is the definition.
(thesaurus)
'a fruit' is the wordplay.
'fruit' becomes 'corn' (corn is a kind of fruit).
'a'+'corn'='ACORN'
'or' acts as a link.
(Other definitions for acorn that I've seen before include "old computer?" , "From which a tall tree may grow" , "Nut of oak tree" , "Model in stone" , "Fruit-seed of the oak" .)