Clear learner to be seen before mid-January by detectives (5)
I believe the answer is:
lucid
'clear' is the definition.
('lucid' can be similar in meaning to 'clear')
'learner to be seen before mid-january by detectives' is the wordplay.
'learner' becomes 'l'.
'to be seen before' is a charade indicator (letters next to each other).
'mid' says to take the centre.
'by' is a charade indicator (letters next to each other).
'detectives' becomes 'cid' (**).
The middle letter of 'january' is 'u'.
'l'+'u'+'cid'='LUCID'
(Other definitions for lucid that I've seen before include "Easy to understand, clearly expressed" , "Quite clear" , "Clear, easily understandable" , "unlike 5 [OPAQUE]" , "between raves?" .)