Final answer:
False, Subcultures are not a threat to the mainstream culture, as they coexist and participate in the larger society.
Step-by-step explanation:
Subcultures are not necessarily a threat to the mainstream culture. Subcultures are smaller cultural groups within a larger culture, and while they may have their own beliefs, behaviors, and customs, they still exist within and participate in the larger society. A subculture can coexist with the mainstream culture without posing a threat to it. In contrast, countercultures actively reject and defy the norms and values of the larger society, and they may pose a threat to the mainstream culture. So, the statement 'By definition, subcultures are a threat to the mainstream culture' is False.