Reserve power needed for small cooler (3,4)
I believe the answer is:
ice cube
'small cooler' is the definition.
(small cube which cools drinks)
'reserve power needed' is the wordplay.
'reserve' becomes 'ice ' (both can mean aloofness).
'power' becomes 'cube' (a mathematical power).
'needed' is a charade indicator (letters next to each other).
'ice'+'cube'='ICE-CUBE'
'for' acts as a link.
(Other definitions for ice cube that I've seen before include "Something frozen" , "Drinks cooler" , "Frozen lump" , "Drink coolant" , "Drink cooler" .)