A sentence refers to the pair of words which is complete in itself. In other words, a sentence contains a subject and a predicate. the predicate in a sentence contains the state or action.
To identify the main clause in a sentence, it is included in the predicate and expresses the action of the subject.
Therefore, in the above sentence, "However one researcher offered the possibility that female basketball players do not have more concussions but the mail players are less likely to report them.", The main verb is "offered" and the subject is "one researcher" who is offering the possibility.