Opposition leader wearing horrible fabric (5)
I believe the answer is:
voile
'fabric' is the definition.
(voile is a kind of fabric)
'opposition leader wearing horrible' is the wordplay.
'leader' indicates taking the first letters.
'wearing' means one lot of letters goes inside another.
'horrible' becomes 'vile' (I've seen this before).
The initial letter of 'opposition' is 'o'.
'o' going within 'vile' is 'VOILE'.
(Other definitions for voile that I've seen before include "Light semi-transparent fabric" , "Sheer fabric" , "fine material" .)