Gertrude finally learning about wickedness in royal castle (8)
I believe the answer is:
elsinore
'royal castle' is the definition.
'elsinore' can be an answer for 'castle' (I've seen this before). I'm not certain of the 'royal' bit.
'gertrude finally learning about wickedness' is the wordplay.
'finally' indicates one should take the final letters.
'learning' becomes 'lore' (both can mean tradition or knowledge).
'about' is an insertion indicator.
'wickedness' becomes 'sin' (synonyms).
The last letter of 'gertrude' is 'e'.
'lore' placed around 'sin' is 'lsinore'.
'e'+'lsinore'='ELSINORE'
'in' is the link.
Can you help me to learn more?
(Other definitions for elsinore that I've seen before include "setting for play" , "Zealand port" , "place in Hamlet" , "Danish port, setting of Shakespeare's Hamlet" , "Hamlet's place" .)