one pair (set) of clothes
Used as a counter for a single set of clothing, typically a full outfit. Often appears in contexts like 一張の着物 or 一張の背広.
He had a new suit made.
一張の着物を箪笥にしまった。
I put away one kimono in the chest.
一着 is a more general counter for one article of clothing, while 一張 specifically refers to a full set or outfit.
Compound of 一 (one) and 張 (counter for sets of clothing). The reading いっちょう is a regular sound change from いちちょう.