One involved in great new hospital procedure (6)
I believe the answer is:
triage
'hospital procedure' is the definition.
(I've seen this before)
'one involved in great new' is the wordplay.
'one' becomes 'i' (Roman numeral).
'involved in' is an insertion indicator.
'new' indicates an anagram.
'great' anagrammed gives 'trage'.
'i' put into 'trage' is 'TRIAGE'.
(Other definitions for triage that I've seen before include "Sorting and allocating aid on the basis of need" , "Selection and allocation for medical aid" , "Hospital reception" , "Hospital procedure" , "Determination of patient priority for treatment" .)