Old tennis ace in the States proving inept (7)
I believe the answer is:
useless
'inept' is the definition.
(thesaurus)
'old tennis ace in the' is the wordplay.
'old' becomes 'us' (I am not sure about this - if you are sure you should believe this answer much more).
'tennis ace' becomes 'seles' (Monica Seles).
'in the' indicates putting letters inside.
'us' going around 'seles' is 'USELESS'.
'states proving' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for useless that I've seen before include "Of no practical application, not worth keeping" , "Unemployable" , "Ineffective" , "Of no practical or worthwhile application" , "Poor" .)