Attacks England overcome by defeats (6)
I believe the answer is:
'attacks' is the definition.
(to be beset by something is to be attacked by it)
'england overcome by defeats' is the wordplay.
'england' becomes 'e'.
'overcome by' means one lot of letters goes inside another.
'defeats' becomes 'bests' (best can mean to defeat or overcome).
'e' inserted inside 'bests' is 'BESETS'.
(Another definition for besets that I've seen is " Pesters".)