Gouda with a difference starts to melt in fish (7)

I believe the answer is:

gourami

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'fish' is the definition.
(I have seen 'Tropical fish ' mean 'gourami' so perhaps 'fish' could also mean 'gourami')

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for gourami that I've seen before include "Brightly coloured aquarium fish" , "Tropical fish" .)

I've seen this clue in The Daily Express.

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