What smoker might have that's not as serious? (7)
I believe the answer is:
lighter
I believe this is a double definition.
'what smoker might have' is the first definition.
(for lighting cigarettes)
'not as serious?' is the second definition.
('light' can be the opposite of 'serious?' so perhaps 'lighter' is reasonable for 'not as serious?')
'that's' acts as a link.
(Other definitions for lighter that I've seen before include "less tonnage" , "carrying less cargo?" , "more blonde" , "The girl (anag.)" , "Open boat - spill" .)