Mates wielding old power (5)
I believe the answer is:
steam
'old power' is the definition.
'mates wielding' is the wordplay.
'wielding' indicates an anagram (I've seen 'wielded' mean this (I've seen this in other clues)).
'mates' is an anagram of 'STEAM'.
(Other definitions for steam that I've seen before include "Vaporous exhalation" , "Travel in liner" , "Kettle emission" , "enthusiasm" , "get very angry" .)