Fairly minor heckler finally shut up (6)
I believe the answer is:
pretty
'fairly' is the definition.
(I've seen this in another clue)
'minor heckler finally shut up' is the wordplay.
'minor' becomes 'petty' (I've seen this in another clue).
'heckler finally' becomes 'r' (I can't justify this - if you can you should give a lot more credence to this answer).
'shut up' is an insertion indicator (inserted letters get shut in or shut up).
'petty' placed around 'r' is 'PRETTY'.
Can you help me to learn more?
(Other definitions for pretty that I've seen before include "Somewhat - attractive" , "chocolate-box" , "Daintily good-looking, maybe as a picture" , "like a kettle of fish" , "comely" .)