The expressions below are not related, but they do have something in common. Can you determine what it is?

A friend indeed
Net profit
Better late than never
Man in the moon
Tarzan the Ape Man
Eternal City

HINT:
The same change can be made to one word in each expression to create a new word.

Answer:

One word in each expression can be changed into a new one by adding or inserting an X:

A friend INDEXED
NEXT profit
Better LATEX than never
MANX in the moon
Tarzan the APEX man
EXTERNAL city 

Credit: NY Times Crossword
Wayne Robert Williams
SOMETHING XTRA