Son with a black mop (4)
I believe the answer is:
swab
'mop' is the definition.
(synonyms)
'son with a black' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'with' becomes 'w' (short for with).
'black' becomes 'b' (as in pencil classifications).
's'+'w'+'a'+'b'='SWAB'
(Other definitions for swab that I've seen before include "Absorbent surgical pad" , "Specimen" , "Cloth for cleaning wounds" , "Clean (a wound)" , "Absorbent pad for cleansing or drying" .)