Raves madly as one declares (5)
I believe the answer is:
avers
'one declares' is the definition.
(I've seen this before)
'raves madly' is the wordplay.
'madly' is an anagram indicator.
'raves' anagrammed gives 'AVERS'.
'as' is the link.
(Other definitions for avers that I've seen before include "Swears" , "States firmly - really raves" , "States positively" , "Declares to be true" , "States to be true" .)