Arsonist is more worthless (7)
I believe the answer is:
lighter
This clue is a double definition.
'arsonist' is the first definition.
'more worthless' is the second definition.
('light' can be similar in meaning to 'worthless')
'is' is the link.
(Other definitions for lighter that I've seen before include "less tonnage" , "Vessel" , "more blonde" , "Barge; flame producer" , "Substitute for match" .)