This is the 684th most frequent Japanese word.
定番
A classic or standard item; a staple.
定番 is used to mean 'a standard or classic choice' in this sentence, referring to a popular, expected option on the menu.
このお店のカレーは定番メニューです。
The curry at this restaurant is a classic menu item.
定番 indicates 'a typical or representative feature' of summer in the context of the sentence.
夏といえば、スイカが定番ですよね。
Speaking of summer, watermelon is a classic choice, right?
定番 expresses 'the usual or standard choice' describing the style of clothing in the sentence.