Rewrite ruling on former respiratory aid (4,4)
I believe the answer is:
iron lung
'former respiratory aid' is the definition.
(type of historical medical machine)
'rewrite ruling on' is the wordplay.
'rewrite' indicates an anagram.
'ruling'+'on'='rulingon'
'rulingon' anagrammed gives 'IRON LUNG'.
(Other definitions for iron lung that I've seen before include "Artificial respiration chamber" , "'Type of metal respirator (4,4)'" , "breather" , "Artificial respirator" , "Rigid case fitting over body for artificial respiration" .)