Troublemaker regularly languid thanks to resistance (8)
I believe the answer is:
agitator
'troublemaker' is the definition.
(agitator is a kind of troublemaker)
'regularly languid thanks to resistance' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'resistance' becomes 'r' (electronics abbreviation).
The alternate letters of 'languid' are 'agi'.
'agi'+'ta'+'to'+'r'='AGITATOR'
(Other definitions for agitator that I've seen before include "Political troublemaker" , "Demagogue" , "One who stirs things up, trouble-maker" , "washing machine" , "One causing trouble" .)