Outrun runner last in marathon (4)
I believe the answer is:
skin
'outrun' is the definition.
(thesaurus)
'runner last in marathon' is the wordplay.
'runner' becomes 'ski' (ski is a kind of runner).
'last in' says to take the final letters.
The final letter of 'marathon' is 'n'.
'ski'+'n'='SKIN'
(Other definitions for skin that I've seen before include "External membrane of body" , "Epidermis - peel" , "Outer surface" , "Surface layer" , "Remove the outer layer" .)