It drains off the water when royal egg is cooked (8)
I believe the answer is:
gargoyle
'it drains off the water' is the definition.
The definition suggests a singular noun which matches the answer.
'royal egg is cooked' is the wordplay.
'is cooked' is an anagram indicator (letters cooked into a new form).
'royal'+'egg'='royalegg'
'royalegg' with letters rearranged gives 'GARGOYLE'.
'when' is the link.
Can you help me to learn more?
(Other definitions for gargoyle that I've seen before include "Spout ending in a grotesquely carved figure" , "Ugly carved figure on a building" , "Grotesque architectural statue" , "Ugly person's" , "Grotesquely carved waterspout" .)