Leftie eating endless leeks smelt awful (6)
I believe the answer is:
reeked
'smelt awful' is the definition.
(to reek is to smell very bad)
'leftie eating endless leeks' is the wordplay.
'leftie' becomes 'red' (red can informally mean a political leftwinger).
'eating' is an insertion indicator.
'endless' indicates the central letters.
The central letters of 'leeks' are 'eek'.
'red' enclosing 'eek' is 'REEKED'.
(Other definitions for reeked that I've seen before include "Smelt strongly" , "Gave off fumes" , "Gave off a strong unpleasant smell" , "Smelt to high heaven" , "Stank" .)