Mammoth is not bound to pachyderm’s middle (6)
I believe the answer is:
mighty
'mammoth' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for mighty that I've seen before include "Powerful - huge" , "Strong, powerful" , "Arrogant people are often described as high and . . . . . ." , "Powerful - exalted" , "Having great strength" .)