Second XI’s source of power (5)
I believe the answer is:
steam
This clue might be a double definition.
'second xi's source' is the first definition.
'steam' can be an answer for 'source' (I have seen 'Source of power' mean 'steam' so perhaps 'source' could also mean 'steam'). I am not sure about the 'second xi's' bit.
'power' is the second definition.
(I've seen this before)
'of' is the link.
Can you help me to learn more?
(Other definitions for steam that I've seen before include "Vapour; energy" , "Type of power" , "Engine driver" , "Sort of water" , "Speed" .)