Loathsome Foreign Office bully oddly ignored (4)
I believe the answer is:
foul
'loathsome' is the definition.
(I've seen this before)
'foreign office bully oddly ignored' is the wordplay.
'foreign office' becomes 'fo'.
'oddly ignored' indicates alternate letters.
The alternating letters of 'bully' are 'ul'.
'fo'+'ul'='FOUL'
(Other definitions for foul that I've seen before include "Offensive, abominable" , "Illegitimate tackle" , "Revolting" , "Violation of a sporting rule" , "Abhorrent" .)