Cool part of an old-fashioned kitchen crumble, taking seconds (3,4)
I believe the answer is:
ice cube
'an old-fashioned kitchen crumble taking seconds' is the definition.
I can't tell whether this defines the answer.
'cool part' is the wordplay.
'cool' becomes 'ice' (icing is a kind of cooling).
'part' becomes 'cube' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ice'+'cube'='ICE-CUBE'
'of' is the link.
Can you help me to learn more?
(Other definitions for ice cube that I've seen before include "Cooling addition to drink" , "Drinks cooler" , "Drink coolant" , "Six-faced frozen cooler" , "Drink cooler" .)