Marathon record broken by Briton at last (4)
I believe the answer is:
long
'marathon' is the definition.
(thesaurus)
'record broken by briton at last' is the wordplay.
'record' becomes 'log' (logging is a kind of recording).
'broken by' indicates putting letters inside.
'at last' says to take the final letters.
The final letter of 'briton' is 'n'.
'log' enclosing 'n' is 'LONG'.
(Other definitions for long that I've seen before include "Thus having been treated by stretcher, perhaps" , "Really want" , "hunger" , "Pant" , "bought in advance" .)