Being no end dirty is serious (4)
I believe the answer is:
grim
'serious' is the definition.
(thesaurus)
'being no end dirty' is the wordplay.
'being no end' means to remove the last letter.
'dirty' becomes 'grimy' (similar in meaning).
'grimy' with its final letter taken away is 'GRIM'.
'is' is the link.
(Other definitions for grim that I've seen before include "looking black" , "(Of humour) black; unappealing, depressing" , "The - Reaper is death" , "Stern, cruel" , "Harshly ironic" .)