Shrub showing decay – sort of green all round (6)
I believe the answer is:
'shrub' is the definition.
(protea is a kind of shrub)
'decay sort of green all round' is the wordplay.
'decay' becomes 'rot' (rot is a kind of decay).
'sort of green' becomes 'pea'.
'all round' is an insertion indicator.
'rot' placed inside 'pea' is 'PROTEA'.
'showing' is the link.
(Other definitions for protea that I've seen before include "S.African flowering shrub" , "South African plant" , "South African shrub" , "South African evergreen" , "Ascend" .)