Carol heading north in Spanish city (4)
I believe the answer is:
leon
'spanish city' is the definition.
(Leon is a city in Spain)
'carol heading north' is the wordplay.
'carol' becomes 'noel' (I've seen this in another clue).
'heading north' says the letters should be written in reverse (in a down clue, letters travel north).
'noel' in reverse letter order is 'LEON'.
'in' acts as a link.
(Other definitions for leon that I've seen before include "- Trotsky, communist" , "Put up Chrissie" .)