Question

is the group of words in bold a phrase or a clause?
although both species are casually classified as \big cats,\ lions are mostly nocturnal predators that hunt together, whereas cheetahs are diurnal predators that hunt alone.
phrase
clause

Explanation:

A clause has a subject and a predicate. "lions" (subject) + "are mostly nocturnal predators" (predicate). A phrase lacks a subject - predicate combination. Here, the bold part has both, so it's a clause.

Answer:

clause