Left union leader with detectives, intelligible (5)
I believe the answer is:
lucid
'intelligible' is the definition.
(I've seen this before)
'left union leader with detectives' is the wordplay.
'leader' indicates taking the first letters.
'with' means one lot of letters go next to another.
'detectives' becomes 'cid' (abbreviation for Criminal Investigation Department).
The initial letters of 'left union' is 'lu'.
'lu'+'cid'='LUCID'
(Other definitions for lucid that I've seen before include "Clear; making sense" , "Clearly expressed and altogether rational" , "Transparent" , "Crystal clear" , "Easy to understand, clearly expressed" .)