Counter saw beside horse sizable negative for Mary (5,6)

'counter saw beside' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'horse sizable negative for mary' is the wordplay.
I cannot quite see how this works, but
'horse' could be 'gg' (geegee or GG is a childish term for a horse) and 'gg' is present in the answer.
'for' could be 'to' (eg both can mean 'in the opinion of') and 'to' is located in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?