Crossword Genius

Warmer, one tucking into narrow fissure? (3,6)

Ross

I believe the answer is:

gas heater

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

'warmer' is the definition.
(gas heater is a kind of warmer)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Another definition for gas heater that I've seen is " Domestic warming appliance".)

I've seen this clue in The Guardian.
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