Official uniform worn by leaders of the opposition (9)
I believe the answer is:
regulator
'official' is the definition.
(regulator is a kind of official)
'uniform worn by leaders of the opposition' is the wordplay.
'uniform' becomes 'regular' ('regular' can be similar in meaning to 'uniform'**).
'worn by' means one lot of letters goes inside another (I've seen this in other clues).
'leaders of' indicates taking the first letters.
The first letters of 'the opposition' is 'to'.
'regular' going around 'to' is 'REGULATOR'.
(Other definitions for regulator that I've seen before include "Controller" , "part of the watch" , "Ofsted perhaps" , "Supervisor" , "Governor" .)