Main plank on the shore (8)
I believe the answer is:
seaboard
'the shore' is the definition.
(seaboard is a kind of shore)
'main plank on' is the wordplay.
'main' becomes 'sea' (the main is an old name for the sea).
'plank' becomes 'board' (synonyms).
'on' is a charade indicator (letters next to each other).
'sea'+'board'='SEABOARD'
(Other definitions for seaboard that I've seen before include "Coastal region" , "The coastline" , "The coast or its outline" , "coastal area" .)