The phrase "a fly on a" is correct and usable in written English.
It is typically used in the expression "a fly on the wall," which refers to someone who observes a situation without being noticed.
Example: "I wish I could be a fly on the wall during their meeting to hear what they really think."
Alternatives: "an unnoticed observer" or "a silent witness".
In what became an iconic image in his lab at least—one of the postdocs, Marcela Chavez, drew a fly on a witch's broom.
I was a fly on a wall observing Mom's patience for following around one of our kids with a spoonful of pasta to get him to eat, listening patiently as one child would try to form new words, teaching them all the names of her pet goats growing up in her village in Greece, singing old, Greek folk songs to lull her grandsons to sleep, and telling me, "Your kids teach you how to be a parent".
Tevez has got away with his mischief making and spectacular transfer requests for one sole reason – he always shot around the Eastlands pitch like a fly on a horse's arse.
Fishing a fly on a dead drift often produces a series of multiple hits -- more taps than hits -- on every cast before a bass gets hooked.
He has also included her hand-painted dressing room doors, which depict scenes from her life and Mrs. Kennedy herself as a fly on a wall.
Against such vast immutability the human struggle takes place on the same scale as the insect movements in the grass, and carnage in the streets is no more than the spider-sucked husk of a fly on a dusty windowsill.
"In the big picture, we're a fly on an elephant's behind," says Golden Gourmet President and Chief Executive Craig Anderson.
Or to use his more colorful metaphor, when it comes to regulating the marketing of colonics, "This is a fly on an elephant's rear end".
A stint in the Government Printing Office enabled him to save enough money to take a flyer on a scheme to raise Angora goats out West.
One day, while on rounds in the hospital, he noticed a flyer on a bulletin board recruiting physicians to join the Armed Forces.
In 2003 Premier took a flyer on a Boston-area startup called Netezza whose specialized box zips through data 15 times as fast as Premier's IBM system, Feierstein says, enabling managers to query away to their heart's content.
