Mates using old form of power (5)
I believe the answer is:
steam
'power' is the definition.
(I've seen this before)
'mates using' is the wordplay.
'using' is an anagram indicator (the letters used in a different way).
'mates' with letters rearranged gives 'STEAM'.
'old form of' is the link.
I am not very happy about this link. 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 "Engine driver" , "letter opener?" , "Form of water" , "Vapour from water at boiling temperature" , "Hot water suspended in air" .)