Rank of those originally involved in investigation (5)
I believe the answer is:
caste
'rank' is the definition.
(I've seen this in another clue)
'those originally involved in investigation' is the wordplay.
'originally' indicates taking the first letters.
'involved in' is an insertion indicator.
'investigation' becomes 'case'.
The first letter of 'those' is 't'.
't' inserted within 'case' is 'CASTE'.
'of' acts as a link.
(Other definitions for caste that I've seen before include "Hindu social rank" , "Social class in India" , "Social division in India" , "Brahmans for instance" , "Hereditary social class among Hindus" .)