It's clear police left uniform on top (5)
I believe the answer is:
lucid
'it's clear' is the definition.
('lucid' can be similar in meaning to 'clear')
'police left uniform on top' is the wordplay.
'police' becomes 'cid'.
'left' becomes 'l' (common abbreviation).
'uniform' becomes 'u' (phonetic alphabet: alpha, bravo, charlie etc.).
'on top' is a charade indicator (letters next to each other) (down clues only).
'l'+'u'='lu'
'cid' after 'lu' is 'LUCID'.
(Other definitions for lucid that I've seen before include "Easily understood; sane" , "Thinking clearly" , "Shining, clearly" , "Clear; making sense" , "Clearly presented and easily understood" .)