A bog therefore will supply decorative plant (8)
I believe the answer is:
acanthus
'decorative plant' is the definition.
'a bog therefore' is the wordplay.
'bog' becomes 'can' (both are slang terms for toilet).
'therefore' becomes 'thus' ('thus' can be a synonym of 'therefore').
'a'+'can'+'thus'='ACANTHUS'
'will supply' is the link.
(Other definitions for acanthus that I've seen before include "Herbaceous shrub" , "Prickly-leaved shrub" , "Ridicule" , "Bush?" , "Leaves carved on a Corinthian column" .)