Lean runner by North American city (6)
I believe the answer is:
skinny
'lean' is the definition.
(similar in meaning)
'runner by north american city' is the wordplay.
'runner' becomes 'ski' (something which 'runs', ie moves quickly on snow).
'by' is a charade indicator (letters next to each other).
'north' becomes 'N' (abbreviation).
'american city' becomes 'ny' (short for New York).
'ski'+'n'+'ny'='SKINNY'
(Other definitions for skinny that I've seen before include "Thin, bony" , "stout? Certainly not!" , "Underfed" , "not having much in the way of meat" , "far from stout!" .)