CHI

CHI is the answer to the crossword clue
[Greek X], [X], and [Universal life force]

As the name of the Greek [Letter before psi], CHI (pronounced like the “ki” in “kite”), is clued as

[X, at times],
[X, to Xanthippe],
or even simply as [X].

Like any other [Greek letter], CHI can arrive through vague clues like

[Frat letter],
[Campus letter],
or [Letter from Greece].

CHI fills in the blank for [“The Sweetheart of Sigma ___”]. The tune grew in popularity from a FRAT [Rush home?] song to become a national hit.

The [Penultimate Greek consonant] is also a part of [___-Rho (religious symbol)].

Via Wikipedia:

The Chi Rho (/ˈkaɪ ˈroʊ/; also known as chrismon) is one of the earliest forms of christogram, formed by superimposing the first two (capital) letters—chi and rho (ΧΡ)—of the Greek word ΧΡΙΣΤΟΣ (Christos) in such a way that the vertical stroke of the rho intersects the center of the chi.

It’s part of Xmas
Brendan Emmett Quigley, Wall Street Journal, January 12, 2007

The crossword answer CHI in Ill., and elsewhere

CHI is also the lead half of [___-Town (the Windy City)], making it the answer for the somewhat illegible [Ill. city].

Like BOS, CHI is a [Sox city]. Other sports clues tie the town to the Cubs, Bears, Bulls, and Blackhawks.

The Boston Public [Actor McBride] was born Kenneth McBride. He adopted the name “Chi” after his hometown, and pronounces it the same way (like “shy”).

Other common clues include: