Dessert vegetable that brightens up the garden (5,3)
I believe the answer is:
'that brightens up the garden' is the definition.
'dessert vegetable' is the wordplay.
'dessert' becomes 'sweet' (synonyms).
'vegetable' becomes 'pea' (pea is a type of vegetable).
(Other definitions for sweet pea that I've seen before include "Climbing garden plant - saw tepee (anag)" , "Garden plant with brightly coloured flowers" , "Climbing plant with fragrant flowers" , "Highly-scented garden climber" , "Climbing plant with pastel-coloured flowers" .)