Reprobate entered age falsely (10)
I believe the answer is:
degenerate
'reprobate' is the definition.
(I know that degenerate is a type of reprobate)
'entered age falsely' is the wordplay.
'falsely' is an anagram indicator.
'entered'+'age'='enteredage'
'enteredage' is an anagram of 'DEGENERATE'.
(Other definitions for degenerate that I've seen before include "dishonourable" , "Dissolute" , "Debauched" , "Corrupt type" , "Rake" .)