Holy water in which an egg's cooked (6)
I believe the answer is:
ganges
'holy water' is the definition.
(I've seen this before)
'an egg's cooked' is the wordplay.
'cooked' indicates an anagram (letters cooked into a new form).
'an'+'eggs'='aneggs'
'aneggs' is an anagram of 'GANGES'.
'in which' acts as a link.
(Other definitions for ganges that I've seen before include "Indian flower" , "Sacred river of N India" , "Holy water" , "Sacred Hindu river in India" , "River flowing into the Bay of Bengal" .)