Freeze ingredient for milk pudding that’s not started (3)
I believe the answer is:
ice
'freeze' is the definition.
(as in water turning into ice)
'ingredient for milk pudding that's not started' is the wordplay.
'ingredient for milk pudding' becomes 'rice' (milk pudding can contain rice).
'that's not started' means to remove the first letter (I've seen 'not start' mean this).
'rice' with its initial letter taken off is 'ICE'.
(Other definitions for ice that I've seen before include "such an age" , "Cubes in a drink" , "Cooler" , "Diamonds, colloquially" , "Top cake" .)