Set of twenty (5)
I believe the answer is:
score
'set of twenty' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for score that I've seen before include "Tally; scratch" , "musical work" , "Orchestrate; twenty" , "Chalk up - twenty" , "Gain a point (20 of them?)" .)