Mark square one (7)
I believe the answer is:
scratch
I believe this clue is a double definition.
'mark' is the first definition.
(I know that mark can be written as scratch)
'square one' is the second definition.
(I've seen this in another clue)
(Other definitions for scratch that I've seen before include "minor injury" , "cash" , "haphazard a team?" , "small injury" , "given no advantage" .)