Declare Vera to be mad (4)
I believe the answer is:
aver
'declare' is the definition.
(averring is a kind of declaring)
'vera to be mad' is the wordplay.
'to be mad' indicates anagramming the letters.
'vera' anagrammed gives 'AVER'.
(Other definitions for aver that I've seen before include "State firmly, perhaps rave" , "State firmly - don't rave" , "State firmly, propound" , "Firmly maintain" , "protest" .)