Glossy small vegetable (5)
I believe the answer is:
sleek
'glossy' is the definition.
('sleek' can be a synonym of 'glossy')
'small vegetable' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'vegetable' becomes 'leek' (leek is a kind of vegetable).
's'+'leek'='SLEEK'
(Other definitions for sleek that I've seen before include "Aerodynamic, streamlined" , "Glossy and well fed" , "Smooth - well-fed" , "with glossy coats" , "Looking prosperous" .)