Courageous person follows criminal group into nerve centre (8)
I believe the answer is:
ganglion
'nerve centre' is the definition.
(a group of nerve cells)
'courageous person follows criminal group' is the wordplay.
'courageous person' becomes 'lion' (I've seen this before).
'follows' means one lot of letters go next to another.
'criminal group' becomes 'gang' (a criminal gang).
'lion' put after 'gang' is 'GANGLION'.
'into' acts as a link.
(Other definitions for ganglion that I've seen before include "centre of interest" , "Collection of nerve cells" , "A longing for group of nerve cells" , "Group of nerve cells from which fibres radiate" , "concentration of strength" .)