Betting system supplying sum before end of race (4)
I believe the answer is:
tote
'betting system' is the definition.
(I've seen this before)
'sum before end of race' is the wordplay.
'sum' becomes 'tot' ('tot' can be a synonym of 'sum'**).
'before' says to put letters next to each other.
'end of' suggests the final letters.
The final letter of 'race' is 'e'.
'tot'+'e'='TOTE'
'supplying' is the link.
(Other definitions for tote that I've seen before include "system for betters" , "Transfer" , "Wield or carry something heavy" , "Horse race betting system" , "Carry (a gun)" .)