Perfect lad in possession of marijuana (4,2)
I believe the answer is:
spot on
'perfect' is the definition.
(I've seen this before)
'lad in possession of marijuana' is the wordplay.
'lad' becomes 'son' (lad can informally mean a young boy).
'in possession of' is an insertion indicator (has inside).
'marijuana' becomes 'pot' (slang term for marijuana).
'son' placed around 'pot' is 'SPOT-ON'.
(Other definitions for spot on that I've seen before include "perfect" , "what soiled garment had?" , "Totally true" , "What the leading actor may have" , "Exact" .)