Plain clothes police essentially ignorant (9)
I believe the answer is:
oblivious
'ignorant' is the definition.
(I know that ignorant can be written as oblivious)
'plain clothes police essentially' is the wordplay.
'plain' becomes 'obvious' (similar in meaning).
'clothes' is an insertion indicator (some letters clothe or cover others).
'essentially' says to take the centre.
The centre of 'police' is 'li'.
'obvious' enclosing 'li' is 'OBLIVIOUS'.
(Other definitions for oblivious that I've seen before include "In ignorance" , "totally in the dark?" , "Forgetful; unaware" , "Blind (to)" , "like Lethe?" .)