NATO is acting strangely hostile (12)
I believe the answer is:
antagonistic
'hostile' is the definition.
(similar in meaning)
'nato is acting strangely' is the wordplay.
'strangely' indicates an anagram.
'nato'+'is'+'acting'='natoisacting'
'natoisacting' is an anagram of 'ANTAGONISTIC'.
(Other definitions for antagonistic that I've seen before include "Actively hostile" , "Ill-disposed" , "Actively opposing - stag in action (anag)" .)