Mostly hostile states (5)
I believe the answer is:
avers
'states' is the definition.
(to aver is to declare or state)
'mostly hostile' is the wordplay.
'mostly' means to remove the last letter.
'hostile' becomes 'averse' (I've seen this in another clue).
'averse' with its last letter taken off is 'AVERS'.
(Other definitions for avers that I've seen before include "Alleges, asserts" , "Swears" , "Declares to be true" , "States to be true" , "States positively" .)