A grain seen only in oak (5)

I believe the answer is:

acorn

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'only in oak' is the definition.
The definition and answer can be both plants as well as being singular nouns.
I also know that 'oak' relates to this answer.

'a grain' is the wordplay.
'grain' becomes 'corn' (corn is a kind of grain).
'a'+'corn'='ACORN'

'seen' acts as a link.

Can you help me to learn more?

(Other definitions for acorn that I've seen before include "form of seeding" , "Nut of oak tree" , "Oak seed" , "Fruit of the oak tree" , "From which a "tall oak" may grow" .)

I've seen this clue in The Globe and Mail.

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