Check covers a source of power (5)
I believe the answer is:
steam
'source of power' is the definition.
(I've seen this before)
'check covers a' is the wordplay.
'check' becomes 'stem' (both can mean to control or stop).
'covers' indicates putting letters inside.
'stem' going around 'a' is 'STEAM'.
(Other definitions for steam that I've seen before include "Engine driver" , "Vapourised water" , "Invisible gas from boiling water" , "What powered old engines" , "Vapour; energy" .)