Prudent girl encrypted secret (8)
I believe the answer is:
discreet
'prudent' is the definition.
(similar in meaning)
'girl encrypted secret' is the wordplay.
'girl' becomes 'di' ().
'encrypted' indicates an anagram.
'secret' anagrammed gives 'screet'.
'di'+'screet'='DISCREET'
(Other definitions for discreet that I've seen before include "Careful to avoid embarrassment" , "Unobtrusive; careful and prudent" , "Showing prudence and circumspection" , "Intentionally unobtrusive" , "Cautious, doesn't give away secrets" .)