Top of black bug (6)
I believe the answer is:
blouse
'top' is the definition.
(I know that blouse is a type of top)
'black bug' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'bug' becomes 'louse' (lice are insects).
'b'+'louse'='BLOUSE'
'of' acts as a link.
(Other definitions for blouse that I've seen before include "Within a building" , "Woman's upper garment" , "Garment like a shirt" , "Woman's "shirt"" , "Top garment - so blue (anag)" .)