I'm grateful petitions for audience exist (6,2)
I believe the answer is:
praise be
'i'm grateful' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'petitions for audience exist' is the wordplay.
'petitions' becomes 'prays' (I've seen this in another clue).
'for audience' indicates a 'sounds like' (homophone) clue (how it might sound to an audience).
'exist' becomes 'be' (synonyms).
'prays' sounds like 'praise'.
'praise'+'be'='PRAISE BE'
Can you help me to learn more?
(Another definition for praise be that I've seen is " hallelujah!".)