A pop bottle a child holds (4)
I believe the answer is:
each
'a pop' is the definition.
(eg 'one pound each' means 'one pound a pop')
'bottle a child holds' is the wordplay.
'holds' indicates the answer is hidden within the clue.
'EACH' can be found hidden inside 'bottle a child'.
(Other definitions for each that I've seen before include "Per person" , "Every one of a number" , "Every - apiece" , "Singly, per capita" , "every one of a group" .)