To detectives, even a half clue can be quite clear (5)
I believe the answer is:
lucid
'be quite clear' is the definition.
(I've seen this before)
'to detectives even a half clue' is the wordplay.
'to detectives even' becomes 'luc' (I am not sure about this - if you are sure you should believe this answer much more).
'a half' indicates taking half.
'clue' becomes 'idea' ('I haven't a clue' means 'I have no idea').
'idea' with half the letters taken is 'id'.
'luc'+'id'='LUCID'
'can' is the link.
Can you help me to learn more?
(Other definitions for lucid that I've seen before include "Clear; making sense" , "Clear, sane" , "Clearly expressed; sane" , "Able to think clearly" , "Intellectually bright" .)