Vessel, part sunken, holds up unsupported (9)
I believe the answer is:
strapless
'unsupported' is the definition.
(not supported by straps)
'vessel part sunken holds up' is the wordplay.
'holds' says the answer is hidden in the clue.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'sselparts' is hidden in the letters of 'vessel part sunken'.
'sselparts' backwards is 'STRAPLESS'.
(Other definitions for strapless that I've seen before include "Daring dress is so" , "lacking support" , "off one's shoulders" , "As dress may be" .)