Mates used old power (5)
I believe the answer is:
steam
'old power' is the definition.
'mates used' is the wordplay.
'used' is an anagram indicator.
'mates' anagrammed gives 'STEAM'.
(Other definitions for steam that I've seen before include "Sort of water" , "Invisible gas from boiling water" , "Vapour from water at boiling point" , "It's water mist, mates" , "Type of power" .)