Article brought in to block power source (5)
I believe the answer is:
steam
'power source' is the definition.
'article brought in to block' is the wordplay.
'article' becomes 'a' (indefinite article).
'brought in' indicates putting letters inside.
'to block' becomes 'stem' (I've seen this before**).
'a' put inside 'stem' is 'STEAM'.
(Other definitions for steam that I've seen before include "Hot water suspended in air" , "Travel in liner" , "12 dn water vapour" , "Water at boiling temperature diffused in the atmosphere" , "Vapour from water at boiling point" .)