Spirit of revolutionary Greek character underpinning resistance (3)
I believe the answer is:
rum
'spirit' is the definition.
(alcoholic spirit)
'revolutionary greek character underpinning resistance' is the wordplay.
'revolutionary' says the letters should be written backwards.
'greek character' becomes 'mu' (letter from the Greek alphabet).
'underpinning' says to put letters next to each other (I've seen this in other clues).
'resistance' becomes 'r' (electronics abbreviation).
'mu' back-to-front is 'um'.
'um' put after 'r' is 'RUM'.
'of' acts as a link.
(Other definitions for rum that I've seen before include "Peculiar like sailor's drink (3)" , "Eerie" , "Alcoholic drink from molasses" , "Sugar-cane spirit" , "Odd spirit from sugar cane" .)