Resolution could be so cruel (7)
I believe the answer is:
closure
'resolution' is the definition.
(I have seen 'Emotional resolution ' mean 'closure' so perhaps 'resolution' could also mean 'closure')
'could be so cruel' is the wordplay.
'could be' indicates an anagram.
'so'+'cruel'='socruel'
'socruel' with letters rearranged gives 'CLOSURE'.
(Other definitions for closure that I've seen before include "sense of satisfaction" , "Feeling of emotional resolution" , "Settlement giving psychological satisfaction" , "End; fastening" , "Co-rules (anag.)" .)