They're heavenly, one thinks (5)

I believe the answer is:

muses

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'one thinks' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for muses that I've seen before include "Nine Greek and Roman goddesses presiding over the arts" , "Meditates on the nine goddesses" , "Nine goddesses who presided over learning and the arts" , "Sources of artists' inspiration" , "Clio, Euterpe, Terpsichore etc" .)

I've seen this clue in the Evening Standard.

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