Ineffective thing gone awry, resentment (7)
I believe the answer is:
dudgeon
'resentment' is the definition.
(I've seen this before)
'ineffective thing gone awry' is the wordplay.
'ineffective thing' becomes 'dud'.
'awry' indicates an anagram.
'gone' anagrammed gives 'geon'.
'dud'+'geon'='DUDGEON'
(Other definitions for dudgeon that I've seen before include "huff" , "Offended indignation" , "indignant state" , "Devil's Disciple" , "feeling of resentment" .)