Setter's second ghost theme? Elgar ultimately promises to be puzzling (10)

I believe the answer is:

mysterious

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'be puzzling' is the definition.
(thesaurus)

'setter's second ghost theme? elgar ultimately promises' is the wordplay.
'setter's second' becomes 'mys' (I can't justify this - if you can you should give a lot more credence to this answer).
'ultimately' suggests the final letters.
'promises' becomes 'IOUs' (promises to pay a debt).
The final letters of 'ghost theme elgar' are 'ter'.
'mys'+'ter'+'ious'='MYSTERIOUS'

'to' acts as a link.

Can you help me to learn more?

(Other definitions for mysterious that I've seen before include "Strange" , "Enigmatic, baffling" , "creating a puzzle" , "Inexplicable" , "Secret" .)

I've seen this clue in The Guardian.

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