Incompetent former tennis player taken in by American (7)
I believe the answer is:
useless
'incompetent' is the definition.
(useless can mean incompetent or ineffective)
'former tennis player taken in by american' is the wordplay.
'former tennis player' becomes 'Seles' (Monica Seles).
'taken in by' means one lot of letters goes inside another.
'american' becomes 'US' (United States).
'seles' placed into 'us' is 'USELESS'.
(Other definitions for useless that I've seen before include "Lacking in purpose" , "Ineffectual" , "Of no practical or worthwhile application" , "Is Sue less impractical?" , "to no avail" .)