Crossword Genius

Cruel person unhappy with one holy person (6)

Ross

I believe the answer is:

sadist

'cruel person' is the definition.
(someone who derives pleasure from inflicting pain)

'unhappy with one holy person' is the wordplay.
'unhappy' becomes 'sad' (similar in meaning).
'with' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'holy person' becomes 'st' (short for saint).
'sad'+'i'+'st'='SADIST'

(Other definitions for sadist that I've seen before include "One deriving pleasure from inflicting pain on others" , "One who delights in cruelty" , "Someone who finds pleasure in hurting others" , "One enjoying inflicting pain" , "Deviant" .)

I've seen this clue in The Guardian.
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