Ring up about one garden shrub (5)
I believe the answer is:
lilac
'one garden shrub' is the definition.
'lilac' can be an answer for 'shrub' (lilac is a kind of shrub). I am unsure of the 'one garden' bit.
'ring up about' is the wordplay.
'ring up' becomes 'lila' (I can't justify this - if you can you should give a lot more credence to this answer).
'about' becomes 'c' (circa).
'lila'+'c'='LILAC'
Can you help me to learn more?
(Other definitions for lilac that I've seen before include "Shrub (but not always this colour)" , "Shade" , "Spiritual knowledge" , "I call (anag)" , "Shrub with fragment purple or white flowers" .)