Being filthy is serious, at the end of the day (5)
I believe the answer is:
grimy
'being filthy' is the definition.
(I've seen this before)
'serious at the end of the day' is the wordplay.
'serious' becomes 'grim'.
'at the end of' suggests the final letters.
The final letter of 'day' is 'y'.
'grim'+'y'='GRIMY'
'is' is the link.
(Other definitions for grimy that I've seen before include "Ingrained with dirt" , "Besmirched" , "grunge" , "smutty" , "filthy" .)