Erratic arching in mortification (7)
I believe the answer is:
chagrin
'mortification' is the definition.
(I know that mortification can be written as chagrin)
'erratic arching' is the wordplay.
'erratic' is an anagram indicator.
'arching' with letters rearranged gives 'CHAGRIN'.
'in' is the link.
(Other definitions for chagrin that I've seen before include "Emotion" , "Disappointment, exasperation" , "Humiliation, mortification" , "irritation?" , "Shame at having failed" .)