Nought drunk by small child? Just a little Coke (4)
I believe the answer is:
toot
'just a little coke' is the definition.
(toot can mean a snort of cocaine)
'nought drunk by small child?' is the wordplay.
'nought' becomes 'o'.
'drunk by' is an insertion indicator.
'small child?' becomes 'tot' (I've seen this before).
'o' going within 'tot' is 'TOOT'.
(Other definitions for toot that I've seen before include "Short sounding of horn" , "Do it on a flute" , "Briefly blow" , "Short sound of horn" , "Sound of a car horn" .)