Grievous fault credited to the pretentious (9)
I believe the answer is:
flatulent
'pretentious' is the definition.
Both the definition and answer are adjectives. Maybe there's an association between them I don't understand?
'grievous fault credited to the' is the wordplay.
I cannot really understand how this works, but
an anagram of 'fault' is 'flatu' which is located in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
The remaining letters 'len' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for flatulent that I've seen before include "Suffering from excessive intestinal gas" , "Suffering from wind or gas" .)