Son spies twitch? The nerve! (7)
I believe the answer is:
sciatic
'nerve' is the definition.
(sciatic nerve is a kind of nerve)
'son spies twitch?' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'spies' becomes 'CIA' (Central Intelligence Agency).
'twitch?' becomes 'tic' (tic is a kind of twitch).
's'+'cia'+'tic'='SCIATIC'
'the' acts as a link.
(Other definitions for sciatic that I've seen before include "Relating to a nerve of the lower back" , "Nerve connected with back pain" , "Largest nerve in the body" , "Relating to the hip or to pain around the body's longest nerve" , "(Of a nerve) in the region of the hip" .)