Big jar, say (5)
I believe the answer is:
great
'big' is the definition.
(I know that big can be written as great)
'jar say' is the wordplay.
'jar' becomes 'grate' (make unpleasant sound).
'say' indicates a 'sounds like' (homophone) clue.
'grate' sounds like 'GREAT'.
(Other definitions for great that I've seen before include "Dickens novel about Pip" , "Of major significance - very good" , "Terrific" , "Peter the -, Russ. tsar" , "Waterbird" .)