Sarah, who heartens Rosalie (3)

I believe the answer is:

sal

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

'sarah' is the definition.
(Sally is a common pet form of Sarah)

'heartens rosalie' is the wordplay.
'SAL' is hidden within the letters.
But, I'm unsure how this is indicated.

'who' is the link.

Can you help me to learn more?

(Other definitions for sal that I've seen before include "Sarah (dim.)" , "- - volatile" , "girl?" .)

I've seen this clue in the Evening Standard.

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