The key words SHORT and LONG in the flat below can be replaced by two words which differ by a single letter that is inserted into the longer word.

I went to a policeman
To ask my way around.
I figured, since the man was SHORT,
His guidance would be sound.

But no - that cop's directions
Turned out completely wrong.
He might have been an officer,
But was he ever LONG!

HINT:
_ _ _ _ _ _ _ _ _
_ _ _ N _ _ _ _ _ _

Answer:

UNIFORMED
UNINFORMED

Credit:
The Enigma
NPL Magazine
December, 2016
GROUCHO