”A sucker for his charm” 點解?

(1) 原文出自 "Lenka - Trouble is a Friend" 的 歌曲......
But he's there in the dark, he's there in my heart
He waits in the wings, he's gotta play a part
Trouble is a friend, yeah trouble is a friend of mine
So don't be alarmed if he takes you by the arm
I roll down the window, I'm a sucker for his charm
Trouble is a friend, yeah trouble is a friend of mine
(2) sucker (n.)
(a) (Slang 俚語) a person who is easily deceived or swindled,
(b) (Slang 俚語) a person who cannot resist something, eg. he's a sucker for fast cars
(3) I'm a sucker for his charm 譯作中文:
我是一個很容易被他的魅力哄騙而又難以自拔的人

I'm a sucker for his charm
我被他那迷人的魅力所蒙騙.
This is not exactly an idiom but frequently used by Americans.
The work "sucker" somtimes meant stupid.
People would say "sucker to me" to indicate - I am fooled by .........

sucker for sth/sb是對某物/某人入迷的人
如：I've always been a sucker for men with green eyes.（我一向對綠眼睛男人著迷）
所以，I'm a sucker for his charm可譯為：我對他的魅力著迷

希望幫到你！

您好，我推薦您一個非常不錯的英語學習網站，里面有很多非常不錯的學習資源，您能從中得到不少幫助：

http://www.hkenglishstudy.info

祝您好運!