Government spook in Jamaica decapitated fish (5)

I believe the answer is:

guppy

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'fish' is the definition.
(guppy is a kind of fish)

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

(Other definitions for guppy that I've seen before include "Tank occupant" , "Small bright WI fish" , "Small fish of northern South America and the Caribbean" , "Aquarium fish" , "Freshwater fish" .)

I've seen this clue in The Independent.

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