Something done one's seen in part of London (6)
I believe the answer is:
action
'something done' is the definition.
(I've seen this before)
'one's seen in part of london' is the wordplay.
'one' becomes 'i' (Roman numeral).
'seen in' is an insertion indicator.
'part of london' becomes 'acton' (I've seen this before).
'i' inserted into 'acton' is 'ACTION'.
(Other definitions for action that I've seen before include "Order to start shooting" , "Accomplishment" , "Carry out" , "Legal proceeding brought by one party against another" , "Not CIA (anag)" .)