Answer: ABIDE
Explanation: The five longest across entries all are one letter away from having a string of five consecutive letters of the alphabet within them. For instance, SUPERSTAR would have PQRST if there were a Q instead of an E, DEIGHTON would have DEFGH if there were an F instead of an I, and so forth. Take these five pairs of the required letter to have this string and the letter this would replace: these are QE, FI, JN, UA, CY. (I almost put KIMNOVAK in here, but I decided I'd rather have repeating strings RST than repeating letters, since the "I" would also have to get replaced in that entry.) Five 5-letter entries in the grid start and end with these letters: QUITE, FIERI, JOANN, UMBRA, and CADDY. From top to bottom, the middle letters of those grid entries spell ABIDE, the meta answer. This word itself could also have a string of five consecutive letters if the I were replaced with a C. (The only two single 5-letter words with this property that I found were ABIDE and ABODE; if we count hyphenated words, NO-PAR could also work.)