Compassion - sentence that can be read in palm (5,4)

I believe the answer is:

heart line

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'read in palm' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'compassion sentence' is the wordplay.
'compassion' becomes 'heart' (I've seen this in another clue).
'sentence' becomes 'line' (I can't explain this - if you can you should give a lot more credence to this answer).
'heart'+'line'='HEART LINE'

'that can be' is the link.

Can you help me to learn more?

I've seen this clue in The Daily Express.

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