A politician impressed by county's glasswort (8)
I believe the answer is:
samphire
'glasswort' is the definition.
(I know that glasswort can be written as samphire)
'a politician impressed by county's' is the wordplay.
'politician' becomes 'MP' (Member of Parliament).
'impressed by' is an insertion indicator.
'county' becomes 'shire' (term for a county, eg Hampshire, Warwickshire etc).
'a'+'mp'='amp'
'amp' going within 'shire' is 'SAMPHIRE'.
(Other definitions for samphire that I've seen before include "coastal plant" , "Edible seaside plant" , "Maritime plant" , "Fleshy-leaf plant" .)