Adversaries from SE1, people hostile, ultimately, to the west (7)
I believe the answer is:
enemies
'adversaries' is the definition.
(I've seen this in another clue)
'se1 people hostile ultimately to the west' is the wordplay.
'se1' becomes 'SEI' (SE1 resembles SEI).
'people' becomes 'men' (men are people).
'ultimately' says to take the final letters.
'to the west' says the letters should be written in reverse (letters written right to left or 'east to west').
The final letter of 'hostile' is 'e'.
'sei'+'men'+'e'='seimene'
'seimene' in reverse letter order is 'ENEMIES'.
'from' acts as a link.
(Other definitions for enemies that I've seen before include "Foes, hostile groups" , "Foes, opponents" , "Foes, adversaries" , "Those we fight" .)