Incompetent tennis ace divides America (7)
I believe the answer is:
useless
'incompetent' is the definition.
(useless can mean incompetent or ineffective)
'tennis ace divides america' is the wordplay.
'tennis ace' becomes 'seles' (Monica Seles).
'divides' is an insertion indicator (inserted letters create a division or opening).
'america' becomes 'US' (United States).
'seles' going within 'us' is 'USELESS'.
(Other definitions for useless that I've seen before include "Without purpose" , "nohoper?" , "Having no benefical function" , "Ineffective" , "Far from impressive" .)