Shakespearean prince wearing underwear in castles (10)
I believe the answer is:
fortinbras
'shakespearean prince' is the definition.
(Prince Fortinbras is a character in Hamlet)
'wearing underwear in castles' is the wordplay.
'wearing' becomes 'in' (as in being 'in' certain clothes).
'underwear' becomes 'bra' (bra is a type of undergarment).
'in' indicates putting letters inside.
'castles' becomes 'forts' (I've seen this before).
'in'+'bra'='inbra'
'inbra' inserted within 'forts' is 'FORTINBRAS'.
(Other definitions for fortinbras that I've seen before include "Character in Hamlet" , "Norwegian prince in Hamlet" , "Military man" , "Norweigian prince in last scene of >Hamlet" .)