A record number of runners in the field (5)
I believe the answer is:
entry
This clue could be a double definition.
'a record' is the first definition.
(eg entry in a list)
'number of runners in the field' is the second definition.
'entry' can be an answer for 'field' (I've seen this before). I'm unsure of the 'number of runners in the' bit.
Can you help me to learn more?
(Other definitions for entry that I've seen before include "Lobby" , "number competing" , "field" , "candidate" , "Gate" .)