Spanish kingdom's Christmas Rising (4)
I believe the answer is:
leon
'spanish kingdom's' is the definition.
(Kingdom of Len)
'christmas rising' is the wordplay.
'christmas' becomes 'noel' (synonyms).
'rising' says the letters should be written in reverse (going up in a down clue).
'noel' in reverse letter order is 'LEON'.
(Other definitions for leon that I've seen before include "- Trotsky, communist" , "Put up Chrissie" .)