Big ___ (London attraction) (3)
I believe the answer is:
ben
'big london attraction' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ben that I've seen before include "The boy from the Scottish mountains" , "Boy for a Scottish mountain" , "He's big in London" , "Second part of 11" , "Mountain peak (Scot.)" .)