Russian leaves something for Santa to fill (7)
I believe the answer is:
cossack
'russian' is the definition.
(I've seen this before)
'leaves something for santa to fill' is the wordplay.
I cannot really see how this works, but
'leaves' could be 'cos' (cos lettuce leaves) and 'cos' is present in the answer.
The remaining letters 'sack' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for cossack that I've seen before include "Member of Russian, Ukrainian people" , "Russian, traditionally horseman of the Steppes" , "One from southern Russia or Ukraine noted for horse riding skills" , "Tsarist cavalryman" , "Russian tribesman famed for horsemanship" .)