Same meaning as 'half-eaten' (食べ掛け), but more colloquial and rougher in tone; refers to food left partially eaten.
机の上に食い掛けの弁当が置いてあった。
There was a half-eaten bento on the desk.
彼は食い掛けで席を立ったので、残りは私が食べた。
He got up leaving his food half-eaten, so I finished the rest.
そんなに汚い言い方ではなく、『食べ掛け』と言ったほうが無難だ。
Instead of that rough expression, it's safer to say '食べ掛け'.