Plant a parliamentarian in county (8)
I believe the answer is:
samphire
'plant' is the definition.
(samphire is a kind of plant)
'a parliamentarian in county' is the wordplay.
'parliamentarian' becomes 'mp'.
'in' indicates putting letters inside.
'county' becomes 'shire' (I've seen this before**).
'a'+'mp'='amp'
'amp' going within 'shire' is 'SAMPHIRE'.
(Other definitions for samphire that I've seen before include "Fleshy-leaf plant" , "Edible seaside plant" , "Maritime plant" , "coastal plant" .)