It's a religion this, no messing about! (6)
I believe the answer is:
shinto
'it's a religion' is the definition.
(Shinto is an example)
'this no messing about' is the wordplay.
'messing about' is an anagram indicator.
'this'+'no'='thisno'
'thisno' anagrammed gives 'SHINTO'.
(Other definitions for shinto that I've seen before include "State religion of Japan until 1945" , "faith" , "Japenese religion revering ancestors" , "Ancestor-worshipping religion of Japan" , "Religious system" .)