Anton lost to a girl (3)
I believe the answer is:
ann
'a girl' is the definition.
(woman's name)
'anton lost to' is the wordplay.
'lost' indicates named letters should be taken away.
'anton' with 'to' taken away is 'ANN'.
(Other definitions for ann that I've seen before include "... Margret or Hathaway" , "Girl's name" , "Girl" , "eg Miss Widdecombe" , "... -Margaret (film actress)" .)