Lose one's rag before long, being facetious (8)
I believe the answer is:
flippant
'facetious' is the definition.
(thesaurus)
'lose one's rag before long' is the wordplay.
'lose one's rag' becomes 'flip' (both can mean to lose one's temper).
'before' is a charade indicator (letters next to each other).
'long' becomes 'pant' (I've seen this before).
'flip'+'pant'='FLIPPANT'
'being' is the link.
(Other definitions for flippant that I've seen before include "Not showing proper seriousness, disrespectful" , "Impudent" , "anything but serious" , "Cheeky" , "showing disrespectful attitude" .)