Hide runner approaching end of marathon (4)
I believe the answer is:
skin
'hide' is the definition.
('skin' can be a synonym of 'hide')
'runner approaching end of marathon' is the wordplay.
'runner' becomes 'ski' (ski is a kind of runner).
'approaching' means one lot of letters go next to another.
'end of' says to take the final letters.
The last letter of 'marathon' is 'n'.
'ski'+'n'='SKIN'
(Other definitions for skin that I've seen before include "Surface layer" , "External surface" , "What keeps you in" , "Hide - flay" , "Remove the outer layer" .)