Mates used old form of power (5)
I believe the answer is:
steam
'power' is the definition.
(thesaurus)
'mates used' is the wordplay.
'used' indicates anagramming the letters.
'mates' anagrammed gives 'STEAM'.
'old form of' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for steam that I've seen before include "12 dn water vapour" , "Travel in liner" , "Vapour from water at boiling temperature" , "Invisible gas from boiling water" , "fume" .)