Crossword Genius

Big fish howls mournfully, listen! (5,5)

Ross

I believe the answer is:

whale shark

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

'big fish' is the definition.
'whale shark' can be an answer for 'fish' (whale shark is a kind of fish). I am unsure of the 'big' bit.

'howls mournfully listen' is the wordplay.
'howls mournfully' becomes 'whales' (I can't explain this - if you can you should believe this answer much more).
'listen' becomes 'hark' (harking is a kind of listening).
'whales'+'hark'='WHALE-SHARK'

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.
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