Declare answer extremely short (4)
I believe the answer is:
aver
'declare' is the definition.
(averring is a kind of declaring)
'answer extremely short' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'extremely short' becomes 'ver' (I've seen this in other clues).
'a'+'ver'='AVER'
(Other definitions for aver that I've seen before include "State; say" , "Vera could declare to be true" , "Maintain - assert - allege" , "Say firmly, rave if necessary" , "claim" .)