Nervous complaint: spies twitch it has introduced (8)
I believe the answer is:
sciatica
'nervous complaint' is the definition.
(medical complaint caused by an irritated nerve)
'spies twitch it has introduced' is the wordplay.
'spies' becomes 'cia' (Central Intelligence Agency).
'twitch' becomes 'tic' (tic is a kind of twitch).
'it' becomes 'sa' (old-fashioned abbreviation for sex appeal).
'has introduced' indicates putting letters inside.
'cia'+'tic'='ciatic'
'ciatic' going inside 'sa' is 'SCIATICA'.
(Other definitions for sciatica that I've seen before include "Lower back and hip pain" , "Type of neuritis" , "Neuralgia of the hip and thigh" , "Intense pain in the leg" , "Neuralgia of the hip region" .)