A subordinate clause is that is dependent on a main clause. It is also known at the dependent clause and contains the subject and the verb. It, on its own, does not form a complete sentence.
In this sentence,
"You shouldn't just throw away you old clothes because you think they're too threadbare to resell"
The subordinate clause is "because you think they're too threadbare to resell". We can move this to the beginning of the sentence, right after the "just" to have it make more sense.
Our final sentence is,
Just because you think they're too threadbare to resell, you shouldn't throw throw away your old clothes.