Score but one (8)
I believe the answer is:
nineteen
'one' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'score' is the wordplay.
I cannot really see how this works, but
'score' could be 'net' (both can mean to obtain or secure) and 'net' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'but' is the link.
Can you help me to learn more?
(Other definitions for nineteen that I've seen before include "Upper age limit for 2 Down" , "Number less than twenty" , "Prime number" , "Number (at the clubhouse?)" , "not quite the full score" .)