Decorate, as a Christmas tree (4)
I believe the answer is:
trim
'decorate as a christmas tree' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for trim that I've seen before include "(Make) neat and tidy" , "Neat - crop" , "Lop unwanted parts off, tree or hair" , "Pare; in good order" , "Prune - spruce" .)