Start off from exchange in London area (7)
I believe the answer is:
wapping
'london area' is the definition.
(Wapping is a district of London)
'start off from exchange' is the wordplay.
I cannot quite see how this works, but
'from' could be 'in' (I've seen this in other clues) and 'in' is found in the answer.
This explanation may well be incorrect...
'in' acts as a link.
(Other definitions for wapping that I've seen before include "part of our capital" , "Murdoch HQ" , "London area" , "using mobile technology?" , "London Docklands district" .)