Some nerve heading off old raider! (4)
I believe the answer is:
axon
'some nerve' is the definition.
(part of a nerve cell)
'heading off old raider' is the wordplay.
'heading off' means to remove the first letter.
'old raider' becomes 'Saxon' (Saxons raided or invaded other medieval peoples).
'saxon' with its first letter removed is 'AXON'.
(Other definitions for axon that I've seen before include "impulsive conductor" , "Part of cell transmitting" , "Disabling headache" , "Bit of a nerve!" , "extension in cell" .)