Twenty known by those in the know? (5)
I believe the answer is:
score
'twenty' is the definition.
(synonyms)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for score that I've seen before include "Shoot a goal for a twenty" , "Cut - and run!" , "Notch; twenty" , "Grade; twenty" , "Reckoning" .)