Adherents shared a strange lack of compassion (15)

I believe the answer is:

hardheartedness

I'm a little stuck... Click here to teach me more about this clue!

'lack of compassion' is the definition.
The definition and answer can be both feelings as well as being singular nouns.
Perhaps you can see a link between them that I can't see?

'adherents shared a strange' is the wordplay.
'a strange' indicates an anagram.
'adherents'+'shared'='adherentsshared'
'adherentsshared' with letters rearranged gives 'HARDHEARTEDNESS'.

Can you help me to learn more?

I've seen this clue in The Irish Times.

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