Talk idly when changing top for fellow getting larger (6)

I believe the answer is:

yatter

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

I can't explain the rest of the clue.

(Other definitions for yatter that I've seen before include "Chatter incessantly (Scots)" , "Talk idly" , "rabbit on endlessly" , "jabber" , "Talk incessantly" .)

I've seen this clue in The Irish Times.

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