Taxing thug? (5)
I believe the answer is:
heavy
I believe this clue is a double definition.
'taxing' is the first definition.
(similar in meaning)
'thug?' is the second definition.
(both can mean a violent or aggressive person)
(Other definitions for heavy that I've seen before include "Weighty - ponderous" , "Oppressive" , "serious!" , "Stodgy; difficult" , "Hard to lift" .)