It may contain a slow-growing shrub (3)
I believe the answer is:
box
'shrub' is the definition.
(box is a kind of shrub)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for box that I've seen before include "Punch" , "Tackle defender" , "Fight; tree; container" , "Engage in pugilism" , "Topiary shrub" .)