State head's involuntary response (8)
I believe the answer is:
domestic
'state' is the definition.
(both can mean relating to a nation)
'head's involuntary response' is the wordplay.
'head's' becomes 'domes'.
'involuntary response' becomes 'tic' (I've seen this before).
'domes'+'tic'='DOMESTIC'
(Other definitions for domestic that I've seen before include "Household servant" , "Violent quarrel" , "Of the household - or person employed there" , "Personal" , "Concerning home affairs" .)