One of those fancy ideas (5)

I believe the answer is:

dream

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

'fancy ideas' is the definition.
'dream' can be an answer for 'fancy' (both can mean a fantasy). I'm not certain of the 'ideas' bit.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for dream that I've seen before include "Armed (anag)" , "Sleeper's vision" , "Nocturnal vision" , "Fantasy; imagine" , "Switch off" .)

I've seen this clue in the Evening Standard.

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