Crossword Genius

Stop a leak containing a stone (4)

Ross

I believe the answer is:

opal

'a stone' is the definition.
(gemstone)

'stop a leak containing' is the wordplay.
'containing' indicates the answer is hidden within the clue.
'OPAL' is hidden within 'stop a leak'.

(Other definitions for opal that I've seen before include "precious jewel" , "Type of gemstone" , "Semi-transparent gemstone" , "Mineral used as a gemstone, of hydrated silica" , "Milky-white mineral" .)

I've seen this clue in The Daily Express.
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