IF CHER COULD TURN BACK TIME…she might want to consider taking back the nasty crack that made an enemy of her good friend and “Burlesque” co-star, singer CHRISTINA AGUILERA! Can you “Believe”?

Ditzy Diva actually told her young pal she might be great on-screen one day – if only she’d take acting lessons. YEEOW! Christina, who actually prides herself on her acting chops, took Cher’s “well intentioned” critique as a slam at her “Burlesque” performance, and the freeze is on.

Talk about GREAT acting! Insiders are still giggling about the pair’s “friendly” interaction while they were out promoting the flick.

Cher joked that with child CHASTITY’s sex change, “Christina is the daughter I no longer ever had!”

Not any longer! Says an insider: “Christina isn’t taking ‘Mom’s’ calls anymore.”