Pack horse seen around States (9)
I believe the answer is:
haversack
'pack' is the definition.
(I know that pack can be written as haversack)
'horse seen around states' is the wordplay.
'horse' becomes 'hack' (hack is a kind of horse).
'seen around' indicates putting letters inside.
'states' becomes 'avers' (to aver is to declare or state).
'hack' going around 'avers' is 'HAVERSACK'.
(Other definitions for haversack that I've seen before include "Item of luggage" , "Carrier" , "Backpack" , "Strong bag slung over the shoulder" .)