Declare less than average (4)

I believe the answer is:

aver

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'declare' is the definition.
(averring is a kind of declaring)

I can't explain the remainder of the clue.

(Other definitions for aver that I've seen before include "State firmly, propound" , "State firmly and positively" , "State firmly, maybe rave" , "Formally assert" , "Declare positively" .)

I've seen this clue in the Evening Standard.

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