Fast runner — he holds a record at first (4)
I believe the answer is:
hare
'fast runner' is the definition.
(I've seen this before)
'he holds a record at first' is the wordplay.
'he' becomes 'hae' (I can't justify this - if you can you should give a lot more credence to this answer).
'holds' is an insertion indicator.
'at first' indicates taking the first letters.
The initial letter of 'record' is 'r'.
'hae' going around 'r' is 'HARE'.
(Other definitions for hare that I've seen before include "Small rabbit-like mammal" , "Leveret" , "Rabbit-like animal" , "Rabbit-like creature" , "'You can't run with the . . . . and hunt with the hounds'" .)