Immobile patient stifling panic when undressed (9)
I believe the answer is:
inanimate
'immobile' is the definition.
(thesaurus)
'patient stifling panic when undressed' is the wordplay.
'patient' becomes 'inmate' (inmate is a kind of patient).
'stifling' indicates putting letters inside (some letters get stifled or contained).
'when undressed' says to take the centre (remove the outside letters which 'dress' the word).
The middle letters of 'panic' are 'ani'.
'inmate' placed around 'ani' is 'INANIMATE'.
(Other definitions for inanimate that I've seen before include "Not endowed with life" , "Dull" , "Lacking life" , "Not living" , "unable to move" .)