British composer, mentally confused, abandoning carnival city (6)

I believe the answer is:

delius

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'british composer' is the definition.
'delius' can be an answer for 'composer' (I've seen this before). I'm not sure about the 'british' bit.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for delius that I've seen before include "English orchestral composer, d. 1934" , "The Walk to the Paradise Garden composer" , "Frederick -, composer of Sea Drift, Appalachia" , "Frederick -, Brigg Fair composer" , "English composer, Frederick (1862-1934)" .)

I've seen this clue in The Independent.

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