There's a lot in a Christmas stocking (4)
I believe the answer is:
mass
'there's a lot' is the definition.
(synonyms)
'in a christmas stocking' is the wordplay.
'in' says the answer is hidden in the clue.
'MASS' is hidden within 'christmas stocking'.
(Other definitions for mass that I've seen before include "Large number of people" , "Throng" , "Bulk - RC service" , "Pound, maybe" , "Accumulation" .)