Trafalgar Square bird (6)
I believe the answer is:
pigeon
'trafalgar square bird' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pigeon that I've seen before include "a visitor to Trafalgar Square" , "Common bird, wild and domesticated" , "run, perhaps" , "Common city bird" , "Dupe" .)