I ring up about a shrub (5)
I believe the answer is:
lilac
'shrub' is the definition.
(I've seen this before)
'i ring up about a' is the wordplay.
'i ring' becomes 'lil' (I've seen this in other clues).
'up' is a reversal indicator (in down clue: letters go upwards).
'about' becomes 'c' (abbreviation for 'circa').
'c'+'a'='ca'
'ca' written backwards gives 'ac'.
'lil'+'ac'='LILAC'
(Other definitions for lilac that I've seen before include "Shrub with fragrant purplish or white flowers" , "Shade" , "Tree with pink blossoms" , "Purplish in colour" , "Pale pinkish-violet" .)