brain trust vs inner circle

brain trust and inner circle both are nouns.

Word	Noun	Adjective	Verb	Adverb
brain trust	Yes	No	No	No
inner circle	Yes	No	No	No

As nouns, inner circle is a hypernym of brain trust; that is, inner circle is a word with a broader meaning than brain trust:

brain trust: an inner circle of unofficial advisors to the head of a government
inner circle: an exclusive circle of people with a common purpose

Other hypernyms of brain trust include camp, clique, coterie, ingroup, pack.

brain trust (noun)	inner circle (noun)
an inner circle of unofficial advisors to the head of a government	an exclusive circle of people with a common purpose