Weird energy harnessed by republic in revolution (5)
I believe the answer is:
eerie
'weird' is the definition.
(both can mean strange or unsettling)
'by republic in revolution' is the wordplay.
'by' becomes 'e' (I can't explain this - if you can you should give a lot more credence to this answer).
'republic' becomes 'Eire' (Irish name for Ireland).
'in revolution' says the letters should be written backwards (letters revolve back).
'eire' back-to-front is 'erie'.
'e'+'erie'='EERIE'
'energy harnessed' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for eerie that I've seen before include "Sinister" , "Spooky, unearthly" , "Spooky, macabre" , "Unnatural" , "Spooky, creepy" .)