Excellent if the petitioner could keep quiet (5)
I believe the answer is:
super
'excellent' is the definition.
(I've seen this before)
'the petitioner could keep quiet' is the wordplay.
I cannot really see how this works, but
'quiet' could be 'p' (music symbol for 'piano', ie soft or quiet) and 'p' is found in the answer.
'petitioner' could be 'suer' (I've seen this in another clue) and 'suer' is found in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
'if' is the link.
Can you help me to learn more?
(Other definitions for super that I've seen before include "Informally great, really up there" , "Great; wonderful" , "First-rate, top-notch" , "Peerless" , "Incomparable" .)