Prudent princess encrypted secret (8)
I believe the answer is:
discreet
'prudent' is the definition.
(similar in meaning)
'princess encrypted secret' is the wordplay.
'princess' becomes 'di' (Princess Diana).
'encrypted' indicates an anagram.
'secret' with letters rearranged gives 'screet'.
'di'+'screet'='DISCREET'
(Other definitions for discreet that I've seen before include "Doesn't give away secrets - can be trusted" , "Cautious, not giving away secrets" , "Tactful and trustworthy" , "Diplomatic" , "Careful in keeping a secret" .)