Inseparable detective leaves unseen (9)
I believe the answer is:
invisible
'unseen' is the definition.
('invisible' can be similar in meaning to 'unseen')
'inseparable detective leaves' is the wordplay.
'inseparable' becomes 'indivisible' ('indivisible' can be similar in meaning to 'inseparable').
'detective' becomes 'di' (Detective Inspector abbreviation).
'leaves' is a deletion indicator.
'indivisible' with 'di' removed is 'INVISIBLE'.
(Other definitions for invisible that I've seen before include "Out of sight" , "Hidden - imperceptible" , "Blind" , "Impossible to see" , "Not able to be seen" .)