Score five boundaries? (6)
I believe the answer is:
twenty
'score' is the definition.
(synonyms)
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 "John B Keane wrote about many young men of this age" , "One score" , "Score, XX" , "No. between 5 and 1 on dartboard" , "previous clue?" .)