Drop line after editor gets upset (8)
I believe the answer is:
decrease
'drop' is the definition.
(I know that drop is a more specific form of the action decline)
'line after editor gets upset' is the wordplay.
'line' becomes 'crease' (synonyms).
'after' is a charade indicator (letters next to each other).
'editor' becomes 'ed' (abbreviation).
'gets upset' shows that the letters should be reversed in order (I've seen 'getting upset' mean this).
'ed' reversed gives 'de'.
'crease' after 'de' is 'DECREASE'.
(Other definitions for decrease that I've seen before include "Make smaller or fewer" , "Grow smaller or less" , "cut back" , "Become smaller or fewer" , "Deminish, lessen" .)