Failure in half of London area? (6)
I believe the answer is:
demise
'failure' is the definition.
(I've seen this in other clues)
'half of london area?' is the wordplay.
'half of' becomes 'demi' (prefix meaning 'half').
'london area?' becomes 'SE' (abbreviation for South East).
'demi'+'se'='DEMISE'
'in' is the link.
(Other definitions for demise that I've seen before include "Passing" , "The end of something" , "Death, collapse" , "Death or termination" , "(Person's) death" .)