Boy consuming cannabis precisely (4,2)
I believe the answer is:
spot on
'precisely' is the definition.
('spot on' can mean exactly right)
'boy consuming cannabis' is the wordplay.
'boy' becomes 'son' (synonyms).
'consuming' indicates putting letters inside.
'cannabis' becomes 'pot' (synonyms).
'son' going around 'pot' is 'SPOT-ON'.
(Other definitions for spot on that I've seen before include "plumb ..." , "Correct" , "Precise" , "What the leading actor may have" , "Quite right" .)