Wide street (6)
'wide street' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for avenue that I've seen before include "Thoroughfare, boulevard" , "Drive" , "Wide street or path to progress" , "Wide (tree-lined) street" , "Broad tree-lined road" .)
(Other definitions for boulevard that I've seen before include "Wide French thoroughfare" , "Broad avenue" , "Broad tree-lined street" , "Wide city road" , "Long, wide avenue" .)