Marches sitter off to get seasonal decoration (9,4)
I believe the answer is:
christmas tree
'seasonal decoration' is the definition.
(I've seen this before)
'marches sitter off' is the wordplay.
'off' indicates an anagram.
'marches'+'sitter'='marchessitter'
'marchessitter' with letters rearranged gives 'CHRISTMAS-TREE'.
'to get' is the link.
(Other definitions for christmas tree that I've seen before include "Decorated fir" , "Seasonal decoration" , "Norway spruce, usually" , "Decorative conifer" .)