Ring up describing island shrub (5)
I believe the answer is:
lilac
'shrub' is the definition.
(I've seen this before)
'ring up describing island' is the wordplay.
'ring' becomes 'call' ('call' can be a synonym of 'ring').
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'describing' is an insertion indicator (describe can mean to form the outline of).
'island' becomes 'i'.
'call' backwards is 'llac'.
'llac' placed around 'i' is 'LILAC'.
(Other definitions for lilac that I've seen before include "Tree - pinkish-purple colour" , "Flowering tree of the olive family" , "Syringa vulgaris, light purple in colour" , "Genus Syringa shrub" , "Light purple - flowering tree" .)