Insanity of head pursuing deterrent theory (7)
I believe the answer is:
madness
'insanity' is the definition.
(madness is a kind of insanity)
'head pursuing deterrent theory' is the wordplay.
'head' becomes 'ness' (ness is a headland).
'pursuing' is a charade indicator (letters next to each other).
'deterrent theory' becomes 'MAD' (Mutually Assured Destruction, nuclear deterrent).
'ness' put after 'mad' is 'MADNESS'.
'of' is the link.
(Other definitions for madness that I've seen before include "Irrationality" , "Utter foolishness" , "hysteria?" , "Folly" , "Insanity" .)