Declares a verse is shortened (5)
I believe the answer is:
avers
'declares' is the definition.
(averring is a kind of declaring)
'a verse is shortened' is the wordplay.
'is shortened' means to remove the last letter.
'verse' with its final letter taken away is 'vers'.
'a'+'vers'='AVERS'
(Other definitions for avers that I've seen before include "Alleges, asserts" , "States positively" , "Declares to be true" , "Swears" , "States firmly - really raves" .)