Top score on board? (6)

I believe the answer is:

twenty

I'm a little stuck... Click here to teach me more about this clue!

'top score' is the definition.
'twenty' can be an answer for 'score' (I've seen this before). I'm not sure about the 'top' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for twenty that I've seen before include "Hundredweights to a ton" , "Dozen plus eight" , "Figure" , "Score, XX" , "No. between 5 and 1 on dartboard" .)

I've seen this clue in the Evening Standard.

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