Marathon followed by top athlete (4,6)
I believe the answer is:
long jumper
'athlete' is the definition.
(I've seen this before)
'marathon followed by top' is the wordplay.
'marathon' becomes 'long' (marathon can mean long or lengthy).
'followed by' means one lot of letters go next to another.
'top' becomes 'jumper' (type of garment worn as a top).
'long'+'jumper'='LONG JUMPER'
(Other definitions for long jumper that I've seen before include "one that can touch his knees?" , "garment that comes down over the thighs?" , "what covers his or her thighs?" , "Athlete" .)