Military leader rejecting old god revolutionised Christmas (8)
I believe the answer is:
napoleon
'military leader' is the definition.
(Napoleon Bonaparte was a military leader)
'rejecting old god revolutionised christmas' is the wordplay.
'rejecting' says the letters should be written in reverse.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'god' becomes 'pan' (Pan is a Greek god).
'revolutionised' is a reversal indicator.
'christmas' becomes 'Noel' (Noel can mean Christmas).
'o'+'pan'='opan'
'opan' in reverse letter order is 'napo'.
'noel' in reverse letter order is 'leon'.
'napo'+'leon'='NAPOLEON'
(Other definitions for napoleon that I've seen before include "Open loan to the French general. strangely" , "Old French coin" , "card game" , "French emperor, d.1821" , "In France, former sovereign" .)