Proverbially smooth advocate? (4)
I believe the answer is:
silk
This clue is a double definition.
'proverbially smooth' is the first definition.
(ref to the phrase 'smooth as silk')
'advocate?' is the second definition.
(silk is an informal term for a lawyer)
(Other definitions for silk that I've seen before include "Fabric obtained from the cocoon of a worm" , "Soft lustrous fabric" , "Fibre of cocoons" , "Ancient double-oared ship could be mire" , "Rich fabric" .)