Leader of Opposition linked to embassy's oversight (8)
I believe the answer is:
omission
'oversight' is the definition.
(omission is a kind of oversight)
'leader of opposition linked to embassy's' is the wordplay.
'leader of' indicates taking the first letters.
'linked to' means one lot of letters go next to another.
'embassy' becomes 'mission' (I've seen this in other clues).
The initial letter of 'opposition' is 'o'.
'o'+'mission'='OMISSION'
(Other definitions for omission that I've seen before include "Deficiency" , "lapse" , "cut?" , "Leaving-out, oversight" , "Something left out or not done" .)