Something cracked, holding up energy supply (5)
I believe the answer is:
equip
'supply' is the definition.
(I know that equip is a more specific form of supply)
'something cracked holding up energy' is the wordplay.
'something cracked' becomes 'quip' (I can't justify this - if you can you should give a lot more credence to this answer).
'holding up' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'energy' becomes 'e' (physics symbol as in E=mcu).
'quip' put after 'e' is 'EQUIP'.
Can you help me to learn more?
(Other definitions for equip that I've seen before include "Accoutre" , "Supply (with kit)" , "Furnish with supplies" , "Pique (anag.)" , "Provide everything that is required'" .)