Incident not beginning in Trafalgar Square (4)
I believe the answer is:
even
'trafalgar square' is the definition.
'even' can be an answer for 'square' (both mean 'owing nothing' as in 'we're all square'). I'm not certain of the 'trafalgar' bit.
'incident not beginning' is the wordplay.
'incident' becomes 'eve' (I can't explain this - if you can you should believe this answer much more).
'beginning' indicates taking the first letters.
The first letter of 'not' is 'n'.
'eve'+'n'='EVEN'
'in' acts as a link.
(Other definitions for even that I've seen before include "Still - quits" , "Level, equal" , "Iron - uniform - flat" , "time of day" , "of consistent quality" .)