Sun fills Bess with joy (12)

I believe the answer is:

blissfulness

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'joy' is the definition.
(thesaurus)

'sun fills bess' is the wordplay.
'sun fills bess' anagrams to 'BLISSFULNESS'.
Though, I'm unsure how this is indicated.

'with' is the link.

Can you help me to learn more?

I've seen this clue in The Irish News.

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