One in eighteen? (3)

I believe the answer is:

tee

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'one' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'in eighteen?' is the wordplay.
'in' says the answer is hidden in the clue.
'TEE' can be found hidden inside 'eighteen'.

Can you help me to learn more?

(Other definitions for tee that I've seen before include "Golfing item" , "Starting place for a golfer" , "Golfer drives from it" , "Golf-peg" , "Golfing equipment" .)

I've seen this clue in The Times and The Sun.

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