Declares English to be hostile (6)
I believe the answer is:
averse
'be hostile' is the definition.
(I've seen this in another clue)
'declares english' is the wordplay.
'declares' becomes 'avers' (averring is a kind of declaring).
'english' becomes 'E' (abbreviation).
'avers'+'e'='AVERSE'
'to' is the link.
(Other definitions for averse that I've seen before include "Strongly opposed (to something)" , "Having a strong dislike" , "Metal rod; card game" , "'Opposed, disinclined (6)'" , "Having a strong opposition (to)" .)