Claudius's successor said to have gone to war in underwear (10)
I believe the answer is:
fortinbras
'claudius's successor said' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'to have gone to war in underwear' is the wordplay.
'to' becomes 'for'.
'have' means one lot of letters go next to another.
'gone to war' becomes 't' (I can't justify this - if you can you should give a lot more credence to this answer).
'underwear' becomes 'bras'.
'for'+'t'+'in'+'bras'='FORTINBRAS'
Can you help me to learn more?
(Other definitions for fortinbras that I've seen before include "Norwegian prince in Hamlet" , "Military man" , "Norweigian prince in last scene of >Hamlet" , "Character in Hamlet" .)