I would ask for something else ,"No,thanks.But I'll take a bit of tea if you have some.

I would ask for something else ,"No,thanks.But I'll take a bit of tea if you have some.
I would ask for something else ,"No,thanks.But I'll take a bit of tea if you have some.

I would ask for something else ,"No,thanks.But I'll take a bit of tea if you have some.
some 在此处是不定代词,指代 some tea.这里是为了避免与上文中 a bit of tea 中的 tea,省略了some tea 中的 tea,some 由形容词转化为不定代词.
本句的意思是：说话人想再要点儿别的东西,对方可能提议让他用某种东西,于是他拒绝道：“不,如果你有茶叶的话我会喝很多的.”

我还要一些东西、不我不要、谢谢、我想要点茶如果你有的话