Just seen after month in part of London (7)
I believe the answer is:
mayfair
'part of london' is the definition.
(I've seen this before)
'just seen after month' is the wordplay.
'just' becomes 'fair' (just can mean fair in the sense of showing justice).
'seen after' says to put letters next to each other.
'month' becomes 'may' (May is an example).
'fair' put after 'may' is 'MAYFAIR'.
'in' is the link.
(Other definitions for mayfair that I've seen before include "Area of west London" , "capital area" , "Go immediately after this" , "Wealthy London district" , "Affluent London area" .)