Be virtuous about starting to water shrub (7)
I believe the answer is:
dogwood
'shrub' is the definition.
(I've seen this before)
'be virtuous about starting to water' is the wordplay.
'be virtuous' becomes 'dogood' (to do good is to act virtuously).
'about' means one lot of letters goes inside another.
'starting to' says to take the initial letters.
The first letter of 'water' is 'w'.
'dogood' going around 'w' is 'DOGWOOD'.
(Other definitions for dogwood that I've seen before include "Flowering shrub" , "tree or shrub" , "timber" , "grows wild" , "Flowering tree" .)