Clear clue uncovered by detectives (5)
I believe the answer is:
lucid
'clear' is the definition.
(similar in meaning)
'clue uncovered by detectives' is the wordplay.
'uncovered' means to look at the middle letters.
'by' is a charade indicator (letters next to each other).
'detectives' becomes 'cid' (abbreviation for Criminal Investigation Department).
The central letters of 'clue' are 'lu'.
'lu'+'cid'='LUCID'
(Other definitions for lucid that I've seen before include "Clearly expressed and altogether rational" , "Sensible" , "Clear-cut" , "Able to think clearly" , "Clear; bright" .)