Lady sleuth entering, her voice shaky for one doing surprisingly well! (12)
I believe the answer is:
overachiever
'one doing surprisingly well' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'lady sleuth entering her voice shaky' is the wordplay.
I cannot really understand how this works, but
'lady' could be 'vera' and 'vera' is found in the answer.
'her' could be 'eve' (I've seen this before) and 'eve' is present in the answer.
This may be the basis of clue (or it may be nonsense).
'for' is the link.
Can you help me to learn more?
(Another definition for overachiever that I've seen is " surprisingly successful student".)