Answer therefore must include prison for plant (8)
I believe the answer is:
acanthus
'plant' is the definition.
(acanthus is a kind of plant)
'answer therefore must include prison' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'therefore' becomes 'thus' ('thus' can be a synonym of 'therefore').
'must include' means one lot of letters goes inside another.
'prison' becomes 'can' (the can is American slang for prison).
'a'+'thus'='athus'
'athus' enclosing 'can' is 'ACANTHUS'.
'for' acts as a link.
(Other definitions for acanthus that I've seen before include "Bush?" , "Growth" , "Plant with prickly leaves" , "Prickly-leaved shrub" , "Prickly-leaved plant" .)