Discourage after rust develops (9)
I believe the answer is:
frustrate
'discourage' is the definition.
(frustrating is a kind of discouraging)
'after rust develops' is the wordplay.
'develops' indicates anagramming the letters.
'after'+'rust'='afterrust'
'afterrust' is an anagram of 'FRUSTRATE'.
(Other definitions for frustrate that I've seen before include "Put a stop to" , "Prevent from progressing" , "'Thwart, disappoint (9)'" , "Thwart or baffle" , "Prevent (plan) from being fulfilled" .)