Involuntary response having half-read cable (6)
I believe the answer is:
reflex
'involuntary response' is the definition.
(involuntary bodily reaction)
'half-read cable' is the wordplay.
'half' indicates taking half.
'cable' becomes 'flex' (type of flexible electric cable).
'read' with half the letters taken is 're'.
're'+'flex'='REFLEX'
'having' is the link.
(Other definitions for reflex that I've seen before include "Automatic instinctive reaction" , "Sneeze, blink or hiccup?" , "Action not requiring conscious thought" , "Knee-jerk" , "Responsive" .)