Mark is cross, twice (5)
I believe the answer is:
score
'mark' is the definition.
(synonyms)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for score that I've seen before include "Nick; twenty" , "musical work" , "Chalk up - twenty" , "Musical composition in written form" , "Tally; 20" .)