A representative visiting county's plant by the sea (8)
I believe the answer is:
samphire
'plant by the sea' is the definition.
(plant which grows by the sea)
'a representative visiting county's' is the wordplay.
'representative' becomes 'mp' (Member of Parliament, a representative).
'visiting' is an insertion indicator.
'county' becomes 'shire' (I've seen this in other clues).
'a'+'mp'='amp'
'amp' inserted into 'shire' is 'SAMPHIRE'.
(Other definitions for samphire that I've seen before include "Fleshy-leaf plant" , "Maritime plant" , "Edible seaside plant" , "coastal plant" .)