Crossword Genius

This seat is uncomfortable for a nap (6)

Ross

I believe the answer is:

siesta

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

This clue might be a double definition.

'this seat is uncomfortable' is the first definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'for a nap' is the second definition.
(siesta is a kind of nap)

Can you help me to learn more?

(Other definitions for siesta that I've seen before include "Afternoon sleep" , "Forty winks" , "Diurnal nap in Spain" , "PM snooze" , "After lunch nap" .)

I've seen this clue in The Globe and Mail.
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