Black perhaps, medium and not as smooth (8)
I believe the answer is:
seamless
'smooth' is the definition.
(similar in meaning)
'black perhaps medium and not' is the wordplay.
'black perhaps' becomes 'sea' (Black Sea is an example).
'medium' becomes 'm' (as in UK clothing sizes).
'and' means one lot of letters go next to another.
'not' becomes 'less' (I've seen this in other clues).
'sea'+'m'+'less'='SEAMLESS'
'as' is the link.
(Other definitions for seamless that I've seen before include "Unbroken" , "Of cloth, made without a join or stitches" , "Without obvious joins" , "Showing no join" , "Without apparent joins" .)