Twenty, but no more in 1 (5)
I believe the answer is:
score
'twenty' is the definition.
(synonyms)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for score that I've seen before include "Keep count" , "Reckoning" , "Number of goals/points" , "Tally; group of twenty" , "Notch; twenty" .)