Stop a leak holding a stone (4)
I believe the answer is:
opal
'a stone' is the definition.
(gemstone)
'stop a leak holding' is the wordplay.
'holding' says the answer is hidden in the clue.
'OPAL' is hidden within 'stop a leak'.
(Other definitions for opal that I've seen before include "Semi-transparent gem" , "Precious stone" , "often semi-transparent" , "Semi-precious stone of b;uish white colour" , "Gem with delicate colours" .)