Pack animal that is a bundle of nerves (8)
I believe the answer is:
ganglion
'is a bundle of nerves' is the definition.
(group of nerve cells)
'pack animal' is the wordplay.
'pack' becomes 'gang' (synonyms).
'animal' becomes 'lion' (I've seen this before).
'gang'+'lion'='GANGLION'
'that' is the link.
(Other definitions for ganglion that I've seen before include "Nerve centre" , "concentration of strength" , "Group of nerve cells from which fibres radiate" , "A longing for group of nerve cells" , "Swelling" .)