Undone by game against Home Counties (5)
I believe the answer is:
loose
'undone' is the definition.
(thesaurus)
'game against home counties' is the wordplay.
'game' becomes 'loo' (historic card game found in Jane Austen novels**).
'against' says to put letters next to each other.
'home counties' becomes 'se'.
'loo'+'se'='LOOSE'
'by' is the link.
(Other definitions for loose that I've seen before include "escaped!" , "Freed" , "Release" , "Showing fast behaviour" , "Unfixed" .)