Small vegetable, smooth in appearance (5)
I believe the answer is:
sleek
'smooth in appearance' is the definition.
(sleek can mean smooth or shiny)
'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 "Pleasingly smooth" , "Well groomed" , "Elegant, streamlined" , "Glossy and well fed" , "with glossy coats" .)