See son with drug (4)
I believe the answer is:
spot
'see' is the definition.
(to spot is to notice or catch sight of)
'son with drug' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'with' means one lot of letters go next to another.
'drug' becomes 'pot' (slang term for cannabis).
's'+'pot'='SPOT'
(Other definitions for spot that I've seen before include "Mark, dot" , "small amount" , "moral flaw" , "make out" , "Notice - place" .)