one's only good outfit
Rare expression for one's sole presentable set of clothes, such as a suit or kimono. The more common synonym is 一帳羅 (いっちょうら).
See also: 一張羅 (いっちょうら)
「一つ着る物」 means what is commonly called 一張羅 (いっちょうら).
Phrase composed of 一つ (one) + 着る (wear) + 物 (thing), literally 'one thing to wear'. The exact historical derivation is uncertain.