Reportedly big jar (5)
I believe the answer is:
grate
'jar' is the definition.
(make unpleasant sound)
'reportedly big' is the wordplay.
'reportedly' indicates a 'sounds like' (homophone) clue.
'big' becomes 'great' ('great' can be similar in meaning to 'big').
'great' is a homophone of 'GRATE'.
(Other definitions for grate that I've seen before include "Reduce to small shreds" , "Irritate - rasp - fireplace" , "Shred; scrape" , "Jar; fireplace" , "Rasp; shred" .)