X-ray unit or back specialist hospital department's information (8)
I believe the answer is:
roentgen
'x-ray unit' is the definition.
(unit measuring exposure to radiation from X-rays)
'or back specialist hospital department's information' is the wordplay.
'back' says the letters should be written in reverse.
'specialist hospital department' becomes 'ENT' (Ear, Nose and Throat department).
'information' becomes 'gen' (gen can informally mean information).
'or' back-to-front is 'ro'.
'ro'+'ent'+'gen'='ROENTGEN'
(Other definitions for roentgen that I've seen before include "Measure of radiation" , "X-ray discoverer" , "German physicist" , "Pioneer in radiation" .)