A grain found in oak (5)
I believe the answer is:
acorn
'in oak' is the definition.
(I know that acorn is part of oak)
'a grain' is the wordplay.
'grain' becomes 'corn' (corn is a kind of grain).
'a'+'corn'='ACORN'
'found' acts as a link.
(Other definitions for acorn that I've seen before include "fruit in a cup" , "Might source" , "Fruit of the oak tree" , "from which something bigger may develop" , "mast" .)