Mark is to withdraw from contest (7)
I believe the answer is:
scratch
'mark' is the definition.
(synonyms)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for scratch that I've seen before include "small injury" , "Receiving no allowance < , ready" , "Mark with something sharp" , "Back out of race" , "Don't play" .)