Marathon record broken by nationalist's leader (4)
I believe the answer is:
long
'marathon' is the definition.
(marathon can mean long or lengthy)
'record broken by nationalist's leader' is the wordplay.
'record' becomes 'log' (logging is a kind of recording).
'broken by' indicates putting letters inside.
'leader' suggests taking the first letters.
The initial letter of 'nationalists' is 'n'.
'log' going around 'n' is 'LONG'.
(Other definitions for long that I've seen before include "extended period" , "to be tall" , "Pant" , "bought in advance" , "Thus having been treated by stretcher, perhaps" .)