Drink nothing during the race (3)
I believe the answer is:
tot
'drink' is the definition.
(a small alcoholic drink)
'nothing during the race' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'during' is an insertion indicator.
'the race' becomes 'tt'.
'o' going inside 'tt' is 'TOT'.
(Other definitions for tot that I've seen before include "Small amount of spirits" , "Calculate" , "Add; young child" , "slug?" , "Add up, aggregate" .)