Mates using old power (5)
I believe the answer is:
steam
'old power' is the definition.
'mates using' is the wordplay.
'using' indicates an anagram (the letters used in a different way).
'mates' with letters rearranged gives 'STEAM'.
(Other definitions for steam that I've seen before include "Energy (colloq.)" , "Vaporised water" , "Invisible gas from boiling water" , "Mates (anag)" , "Pre-digital" .)