American exactitude shown in stiff feature (5)
I believe the answer is:
I believe this clue is a double definition.
'american exactitude' is the first definition.
(US spelling of 'rigour')
'stiff feature' is the second definition.
(rigor mortis is shown by a body or 'stiff')
'shown in' acts as a link.
(Other definitions for rigor that I've seen before include "showing failure to respond" , "Stiff, irresponsive state" , "Stiffness (after death?)" , "Muscular cramp" , "Shivering chill before fever" .)