Smartened up a tree by the first of December (7)
I believe the answer is:
spruced
'smartened up' is the definition.
The definition suggests a past participle verb which matches the answer.
'a tree by the first of december' is the wordplay.
I cannot quite understand how this works, but
'tree' could be 'spruce' (spruce is a kind of tree) and 'spruce' is found within the answer.
'first of december' could be 'd' (1st letter of 'december') and 'd' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for spruced that I've seen before include "freshened" , "Did tart" .)