Troublemaker revised brief on foreign currency (9)
I believe the answer is:
firebrand
'troublemaker' is the definition.
(I know that firebrand is a type of troublemaker)
'revised brief on foreign currency' is the wordplay.
'revised' indicates an anagram.
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'foreign currency' becomes 'rand' (I've seen this before).
'brief' anagrammed gives 'fireb'.
'fireb'+'rand'='FIREBRAND'
(Other definitions for firebrand that I've seen before include "Agitator" , "Troublemaker" , "Energetic person often fomenting trouble" , "One who stirs up trouble" , "Rabble-rouser" .)