Small drug advertisement (4)
I believe the answer is:
spot
'advertisement' is the definition.
(I've seen this in another clue)
'small drug' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'drug' becomes 'pot' (slang term for cannabis).
's'+'pot'='SPOT'
(Other definitions for spot that I've seen before include "See; place" , "Lady Macbeth: ''Out, damned . . . .!''" , "See - blemish" , "clock" , "Observe - locality" .)