The old nag, it ran an awful race (5,8)
I believe the answer is:
grand national
'race' is the definition.
(Grand National is an example)
'the old nag it ran an awful' is the wordplay.
'awful' indicates an anagram.
'old'+'nag'+'it'+'ran'+'an'='oldnagitranan'
'oldnagitranan' with letters rearranged gives 'GRAND NATIONAL'.
(Other definitions for grand national that I've seen before include "Famous steeplechase" , "Annual Aintree race" , "Annual steeplechase at Aintree" , "Horse race" , "annual competition" .)