Evening wear isn't practical for working in.

Literal

Evening-dress [topic-は] work [obj-を] do-[for-には] inconvenient [copula-だ].

夜会服 ('evening wear, evening gown') is a compound: 夜会 ('evening party, ball') + 服 ('clothing'). A formal word for high-society dress attire. ~には purposive ('for (doing) X'), negated — 'inconvenient for doing X.'