Major, often in a hole (3)
I believe the answer is:
key
'major' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for key that I've seen before include "Of crucial importance" , "Vital" , "Crucially important" , "It opens the door" , "Clue" .)