America’s female tennis legend becoming incompetent (7)
I believe the answer is:
useless
'incompetent' is the definition.
(I've seen this before)
'america's female tennis legend' is the wordplay.
I cannot really understand how this works, but
'america's' could be 'us' ('US' can be a synonym of 'america') and 'us' is found in the answer.
This explanation may well be incorrect...
'becoming' acts as a link.
(Other definitions for useless that I've seen before include "Unemployable" , "Not practical or constructive" , "Vain, impractical" , "Ineffective" , "Ineffectual" .)