Notice explosives, say, as coastal features (9)

I believe the answer is:
seashells
'notice explosives' is the definition.
Although both the answer and definition are plural nouns, I don't understand how they can define each other.
'as coastal features' is the wordplay.
I cannot really understand how this works, but
'as' is within the answer.
This may be the basis of the clue (or it may be nonsense).
'say' is the link.
Can you help me to learn more?
(Other definitions for seashells that I've seen before include "''She sells ... on the seashore'' (tongue-twister)" , "Coverings of marine organisms" , "Protective homes for salt water molluscs" , "Hard outer coverings of salt water molluscs" , "She might sell them on the seashore" .)
