Clause or phrase that is inserted into the flow of the text and does not directly deal with the current topic is called a expression.

The correct anser is parenthetical expression.

Explanation:

A parenthetical expression refers to a word or group of words added to a sentence, without changing the meaning of such sentence. In other words, a parenthetical expression gives extra but not essential information into a sentence therefore, it could be either added or removed from a particular sentence and it would not change its meaning.

Bearing in mind the stated above, it could be said that a clause or phrase that is inserted into the flow of the text and does not directly deal with the current topic is called a  parenthetical expression.

and your question what topic are you in trouble with?

explanation:

it refers to the definition of full-contact sports, where the condition is that the sport must have athletes must "purposely collide with each other