It's a figment of one's imagination hearing hymn from Cockney in California (7)

I believe the answer is:

chimera

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'it's a figment' is the definition.
(thesaurus)

I can't explain the remainder of the clue.

(Other definitions for chimera that I've seen before include "Vain, foolish, or incongruous fancy" , "Idle fancy" , "Creature such as a gryphon" , "Each rim (anag) - foolish fancy" , "Fire-breathing female monster" .)

I've seen this clue in the Irish Times.

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