Decay in small green vegetable or shrub (6)
I believe the answer is:
protea
'shrub' is the definition.
(protea is a kind of shrub)
'decay in small green vegetable' is the wordplay.
'decay' becomes 'rot' (rot is a kind of decay).
'in' means one lot of letters goes inside another.
'small green vegetable' becomes 'pea'.
'rot' placed within 'pea' is 'PROTEA'.
'or' acts as a link.
(Other definitions for protea that I've seen before include "South African evergreen" , "South African shrub" , "S.African flowering shrub" , "South African plant" , "S. African shrub; to pear (anag.)" .)