Lighter aquatic mammal lacking height, unknown previously (5)

I believe the answer is:

zippo

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'lighter' is the definition.
(I know that Zippo is a type of lighter)

I can't explain the rest of the clue.

(Other definitions for zippo that I've seen before include "lighter!" , "Not a bit" , "duck?" .)

I've seen this clue in The Sunday Times and The Times.

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