Runner is more self-satisfied going round lake (8)
I believe the answer is:
smuggler
'runner' is the definition.
('smuggler' can be a synonym of 'runner')
'more self-satisfied going round lake' is the wordplay.
'more self-satisfied' becomes 'smugger' ('smug' can be a synonym of 'self-satisfied').
'going round' indicates putting letters inside.
'lake' becomes 'l'.
'smugger' enclosing 'l' is 'SMUGGLER'.
'is' acts as a link.
(Other definitions for smuggler that I've seen before include "Contrabandist" , "Runner" , "Illegal importer" , "Exciseman's foe" , "One who imports or exports without paying duties" .)