Didn't accept immature setter, say, since second editor intervenes (6,2)
I believe the answer is:
passed up
'didn't accept' is the definition.
(to pass up on something is to turn it down)
'immature setter say since second editor intervenes' is the wordplay.
'immature setter say' becomes 'pup' (setter is a type of dog).
'since' becomes 'as' (I've seen this in other clues).
'second' becomes 's' (common abbreviation).
'editor' becomes 'ed'.
'intervenes' means one lot of letters goes inside another.
'as'+'s'+'ed'='assed'
'pup' placed around 'assed' is 'PASSED UP'.
(Another definition for passed up that I've seen is " ignored".)