Wager London area included plague (5)
I believe the answer is:
beset
'plague' is the definition.
(I know that plague can be written as beset)
'wager london area included' is the wordplay.
'wager' becomes 'bet' (synonyms).
'london area' becomes 'SE' (abbreviation for South East).
'included' indicates putting letters inside.
'bet' enclosing 'se' is 'BESET'.
(Other definitions for beset that I've seen before include "Surround (with problems)" , "Beleaguer" , "Press" , "Surround and harass" , "Invested" .)