Unable to part, detective leaves unseen (9)
I believe the answer is:
invisible
'unseen' is the definition.
(similar in meaning)
'unable to part detective leaves' is the wordplay.
'unable to part' becomes 'indivisible'.
'detective' becomes 'di' (Detective Inspector abbreviation).
'leaves' suggests deleting specific letters.
'indivisible' with 'di' taken out is 'INVISIBLE'.
(Other definitions for invisible that I've seen before include "Imperceptible by the eye" , "Blind" , "Not able to be seen" , "Impossible to see" , "Out of sight" .)