Grass brand right for blow (8)
I believe the answer is:
haymaker
'blow' is the definition.
(haymaker is a kind of blow)
'grass brand right' is the wordplay.
'grass' becomes 'hay' (I have seen 'Mown grass ' mean 'hay' so perhaps 'grass' could also mean 'hay').
'brand' becomes 'make' ('make' can be a synonym of 'brand').
'right' becomes 'r' (common abbreviation).
'hay'+'make'+'r'='HAYMAKER'
'for' is the link.
(Other definitions for haymaker that I've seen before include "One profiting from summer weather" , "required for one working in the sun?" , "Wild, swinging punch (slang)" , "I was really enjoying myself" , "strong punch" .)