Chief of the bunch? (3)
I believe the answer is:
key
'chief' 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 "Device for lock" , "Piano lever" , "Lock undoer" , "Lock-opener" , "Crucial arch-stone" .)