Germanic invader beheaded – some nerve! (4)
I believe the answer is:
axon
'some nerve' is the definition.
(part of a nerve cell)
'germanic invader beheaded' is the wordplay.
'germanic invader' becomes 'saxon' (I've seen this before).
'beheaded' means to remove the first letter (the head of the word is removed).
'saxon' with its initial letter removed is 'AXON'.
(Other definitions for axon that I've seen before include "Bit of a nerve!" , "Disabling headache" , "extension in cell" , "Part of cell transmitting" , "impulsive conductor" .)