Crossword puzzle clue "ones who can barely be seen"

Ones who can barely be seen?

One barely seen!

Spiro who wrote, "If you've seen one city slum, you've seen them all"

Springsteen "If you've ever seen ___ trick pony then you've seen me"

Feeling that you've seen something before seen something before

"If you've ___ seen a one-trick pony, then you've seen me"

"If you haven't seen ___, you haven't seen New York" (old ad slogan)

Something often seen on a street corner, briefly ... or, literally, something seen in each corner of this puzzle