Demo going round obscure London landmark (6,4)
I believe the answer is:
marble arch
'london landmark' is the definition.
(landmark in London)
'demo going round obscure' is the wordplay.
'demo' becomes 'march' (demonstration).
'going round' means one lot of letters goes inside another (some letters go around others).
'obscure' becomes 'blear' (I've seen this in another clue).
'march' placed around 'blear' is 'MARBLE ARCH'.
(Other definitions for marble arch that I've seen before include "London sight" , "John Nash-designed landmark at northeast corner of Hyde Park" , "tube station" , "national landmark" , "Capital's landmark" .)