premise: A young man in blue jeans and an undershirt dances with a bare-midriffed young woman in a crowded nightclub. hypothesis: A casually dressed man and woman dance in a club.
entailment: Blue jeans and an undershirt are things that are widely known to be worn by people who are casually dressed - we already know the man is casually dressed because he is described as wearing these things. A "club" is another word for a "nightclub".