Untidy dining room at start of year (5)
I believe the answer is:
messy
'untidy' is the definition.
(synonyms)
'dining room at start of year' is the wordplay.
'dining room' becomes 'mess' (mess is a kind of dining room).
'at' says to put letters next to each other.
'start of year' becomes 'y'.
'mess'+'y'='MESSY'
(Other definitions for messy that I've seen before include "Untidy, dirty" , "Dirty and disorderly" , "Disorganised" .)