Sign of future king's personal honour (5)
I believe the answer is:
crown
'honour' is the definition.
(crown is a kind of honour)
'sign of future king's personal' is the wordplay.
'sign of future king' becomes 'cr' (abbreviation for Charles Rex, as in Prince Charles).
'personal' becomes 'own' ('own' can be similar in meaning to 'personal').
'cr'+'own'='CROWN'
(Other definitions for crown that I've seen before include "repair for damaged tooth" , "peak" , "Headdress, coin" , "The sovereign" , "Highest part; coin" .)