Excellent drug lad’s brought round (4-2)
I believe the answer is:
spot-on
'excellent' is the definition.
(I've seen this before)
'drug lad's brought round' is the wordplay.
'drug' becomes 'pot' (slang term for cannabis).
'lad' becomes 'son' (lad can informally mean a young boy).
'brought round' is an insertion indicator.
'pot' placed within 'son' is 'SPOT-ON'.
(Other definitions for spot-on that I've seen before include "Precisely what's required" , "Informally - exactly so, precisely" , "Informally precise, receiving no post" , "'Exactly right (4,2)'" , "Informal expression for precisely" .)