Keep speaking with a painter for so long (8)

I believe the answer is:

sayonara

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'for so long' is the definition.
(synonyms)

'keep speaking with a painter' is the wordplay.
'keep speaking with' becomes 'sayon' (I can't justify this - if you can you should believe this answer much more).
'painter' becomes 'RA' (member of the Royal Academy of Arts).
'sayon'+'a'+'ra'='SAYONARA'

Can you help me to learn more?

(Other definitions for sayonara that I've seen before include "Goodbye (Jap.)" , "Goodbye or farewell in Japan" , "So long" , "Japanese valediction" , "Japanese goodbye" .)

I've seen this clue in The Times.

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