Tired words ruined day at last (6)
I believe the answer is:
drowsy
'tired' is the definition.
(I've seen this before)
'words ruined day at last' is the wordplay.
'ruined' indicates anagramming the letters.
'at last' indicates one should take the final letters.
The final letter of 'day' is 'y'.
'words' is an anagram of 'drows'.
'drows'+'y'='DROWSY'
(Other definitions for drowsy that I've seen before include "Being rather tired" , "Not being very alert" , "Sleepy, somnolent" , "Dull" , "Disinclined to stay awake" .)