The sole effect of using fossil fuels? (6,9)
I believe the answer is:
carbon footprint
'the sole effect' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'using fossil fuels?' is the wordplay.
I cannot quite see how this works, but
'using' could be 'on' (someone using drugs is 'on' them) and 'on' is found in the answer.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for carbon footprint that I've seen before include "emission level" , "personal effects?" , "From measure of energy usage" , "emissions" .)