Enormous jar, so it's said (5)
I believe the answer is:
great
'enormous' is the definition.
(I've seen this before)
'jar so it's said' is the wordplay.
'jar' becomes 'grate' (make unpleasant sound).
'so it's said' shows a homophone (sound like).
'grate' sounds like 'GREAT'.
(Other definitions for great that I've seen before include "Large, important" , "Waterbird" , "Dickens novel about Pip" , "Big, large" , "Chief - weighty" .)