Sit with Santa's new helper (9)
I believe the answer is:
assistant
'helper' is the definition.
('assistant' can be a synonym of 'helper')
'sit with santa's new' is the wordplay.
'with' says to put letters next to each other.
'new' indicates anagramming the letters.
'sit' put after 'santas' is 'santassit'.
'santassit' anagrammed gives 'ASSISTANT'.
(Other definitions for assistant that I've seen before include "Attendant, helper" , "One helping" , "help required?" , "Deputy, subordinate" , "one helps out" .)