Clear case first of investigators dropped (6)
I believe the answer is:
patent
'clear' is the definition.
(synonyms)
'case first of investigators dropped' is the wordplay.
'case' becomes 'patient' (a medical case).
'first of investigators' becomes 'i' (1st letter of 'investigators').
'dropped' is a deletion indicator (some letters are excluded or dropped).
'patient' with 'i' removed is 'PATENT'.
(Other definitions for patent that I've seen before include "Manifest" , "Open the right to an invention" , "It's registered to protect an invention" , "The invention protection is obvious" , "Registration of ownership of invention" .)