Bad language is ultimately exhausting (8)
I believe the answer is:
swearing
'bad language' is the definition.
(thesaurus)
'is ultimately exhausting' is the wordplay.
'ultimately' says to take the final letters.
'exhausting' becomes 'wearing' ('wearing' can be a synonym of 'exhausting').
The last letter of 'is' is 's'.
's'+'wearing'='SWEARING'
(Other definitions for swearing that I've seen before include "Expressing annoyance" , "Vowing" , "Taking an oath" , "blue emissions?" , "Foul language" .)