Lad keeping grass exactly right (4,2)
I believe the answer is:
spot-on
'exactly right' is the definition.
(I've seen this before)
'lad keeping grass' is the wordplay.
'lad' becomes 'son' (lad can informally mean a young boy).
'keeping' means one lot of letters goes inside another (some letters kept inside).
'grass' becomes 'pot' (cannabis).
'son' enclosing 'pot' is 'SPOT-ON'.
(Other definitions for spot-on that I've seen before include "On the button, precisely so" , "Informally precise - no post required" , "Absolutely correct - very accurate" , "Informal expression for precisely" , "Informally - exactly so, precisely" .)