Crossword Genius

Fish found in coastal area when sun goes down (8)

Ross

I believe the answer is:

seahorse

I'm a little stuck... Click here to teach me more about this clue!

'sun goes down' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'fish found in coastal area' is the wordplay.
'fish' becomes 'sea fish' (sea fish is a kind of fish).
'found in coastal area' becomes 'horse' (I can't justify this - if you can you should give a lot more credence to this answer).
'sea'+'horse'='SEAHORSE'

'when' is the link.

Can you help me to learn more?

(Other definitions for seahorse that I've seen before include "marine creature" , "Fish" , "swimmer" , "that's found in the main" .)

I've seen this clue in The Independent.
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