Stayed and spoke at length on a subject (5)

I believe the answer is:

dwelt

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

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

(Other definitions for dwelt that I've seen before include "Focused anxiously" , "Thought moodily (about something)" , "Persistently thought" , "Had house" , "I dreamt I . . . in marble halls (song)" .)

I've seen this clue in The Globe and Mail.

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