Like to run, not on the flat (6)
I believe the answer is:
aslope
'not on the flat' is the definition.
(sloped rather than flat)
'like to run' is the wordplay.
'like' becomes 'as'.
'to run' becomes 'lope' (loping is a kind of running).
'as'+'lope'='ASLOPE'
(Other definitions for aslope that I've seen before include "not on the level" , "italic" , "Not flat" , "Leaning" , "Inclined" .)