Being depressed is, to many, perversity (6)
I believe the answer is:
sadism
'perversity' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'being depressed is to many' is the wordplay.
'being depressed' becomes 'sad' (I've seen this in another clue).
'to many' becomes 'm' (Roman numeral for a thousand).
'sad'+'is'+'m'='SADISM'
Can you help me to learn more?
(Other definitions for sadism that I've seen before include "Pleasure obtained by inflicting pain on others" , "The taking of pleasure in the infliction of pain" , "Perverse pleasure" , "Sexual pleasure from hurting others" , "Deliberate cruelty" .)