Correct relative inhaling cannabis (4,2)
I believe the answer is:
spot on
'correct' is the definition.
(I've seen this before)
'relative inhaling cannabis' is the wordplay.
'relative' becomes 'son' (son is a kind of relative).
'inhaling' is an insertion indicator.
'cannabis' becomes 'pot' ('pot' can be a synonym of 'cannabis').
'son' placed around 'pot' is 'SPOT-ON'.
(Other definitions for spot on that I've seen before include "Quite right" , "Completely accurate" , "Exact" , "Excellent" , "perfect" .)