Evangelist depressed by lower prices (4,4)
I believe the answer is:
mark down
'lower prices' is the definition.
'evangelist depressed by' is the wordplay.
'evangelist' becomes 'mark'.
'depressed' becomes 'down' ('down' can be a synonym of 'depressed').
'by' means one lot of letters go next to another.
'mark'+'down'='MARK-DOWN'
(Other definitions for mark down that I've seen before include "Score points off" , "Lower cost" , "Reduce in price" .)