Bay in a large coastal area (6)
I believe the answer is:
alcove
'bay' is the definition.
(synonyms)
'a large coastal area' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'coastal area' becomes 'cove' (area by the coast).
'a'+'l'+'cove'='ALCOVE'
'in' is the link.
(Other definitions for alcove that I've seen before include "sheltered spot" , "bay" , "Recessed part of a room" , "Recess in a small room or wall" , "A small recess or niche" .)