Declares what is in haversack (5)
I believe the answer is:
avers
'declares' is the definition.
(averring is a kind of declaring)
'in haversack' is the wordplay.
'in' says the answer is hidden in the clue.
'AVERS' can be found hidden inside 'haversack'.
'what is' is the link.
(Other definitions for avers that I've seen before include "Alleges, asserts" , "States positively" , "Swears" , "Declares to be true" , "States firmly - really raves" .)