Seaside fanatic, we hear, that's brown dressed in shell suit (8)
I believe the answer is:
beechnut
'we hear that's brown dressed in shell suit' is the definition.
I can't judge whether this defines the answer.
'seaside fanatic' is the wordplay.
'seaside' becomes 'beech' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fanatic' becomes 'nut' (I've seen this before).
'beech'+'nut'='BEECHNUT'
Can you help me to learn more?
(Other definitions for beechnut that I've seen before include "Fruit of the Fagus tree" , "Fruit of a forest tree" .)