PEN
The word is pennant. It means a flag that denotes a sports achievement or a tapering flag flown on a ship.
Pennant (a tapering flag on a ship).
Ria
A long narrow cut could be called a gash or a slash, depending on the context or severity of the wound.
Recent
a long narrow house
A long narrow cut is called a slit.
The word is ria. It means a long, narrow inlet.
narrow. Also long.
No, the word "flag" does not have a long vowel sound. The "a" in "flag" is pronounced with a short vowel sound.
The word you are looking for is "trench." Trenches are long and narrow depressions in the seafloor characterized by very steep sides.