Declare to be less than average (4)
I believe the answer is:
aver
'declare' is the definition.
(averring is a kind of declaring)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for aver that I've seen before include "Vera could declare to be true" , "Declare positively" , "Profess" , "Declare to be the case" , "Prove to be true" .)