Disorder puts Emergency Room in endless peril (7)
I believe the answer is:
derange
'disorder' is the definition.
(I know that derange is a more specific form of disorder)
'emergency room in endless peril' is the wordplay.
'emergency room' becomes 'ER' (abbreviation popularized by US TV series).
'in' is an insertion indicator.
'endless' means to remove the last letter.
'peril' becomes 'danger' (synonyms).
'danger' with its last letter taken off is 'dange'.
'er' going within 'dange' is 'DERANGE'.
'puts' acts as a link.
(Other definitions for derange that I've seen before include "Remove from order" , "Upset" , "Make mad" , "Drive mad" , "Madden" .)