Stout-hearted friend - but stony! (4)

I believe the answer is:

opal

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'but stony' is the definition.
I can't judge whether this defines the answer.

'stout-hearted friend' is the wordplay.
'hearted' indicates the central letters.
'friend' becomes 'pal' (pal can mean a friend).
The central letter of 'stout' is 'o'.
'o'+'pal'='OPAL'

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(Other definitions for opal that I've seen before include "raw material" , "Opaque mineral used a gemstone" , "Gem stone" , "Lustrous stone" , "Milky-white mineral" .)

I've seen this clue in the Evening Standard.

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