Weaker one starts a fire (7)
I believe the answer is:
lighter
'weaker' is the definition.
('light' can be a synonym of 'weak')
'one starts a fire' is the wordplay.
I cannot quite see how this works, but
'one' could be 'li' (I've seen this in other clues) and 'li' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for lighter that I've seen before include "Barge; igniter" , "craft" , "Barge - alternative to match" , "Vessel" , "Barge; one needed by smoker" .)