Left turn, to detectives, is easy to understand (5)
I believe the answer is:
lucid
'easy to understand' is the definition.
(I've seen this in another clue)
'left turn to detectives' is the wordplay.
'left' becomes 'l' (common abbreviation).
'turn' becomes 'U' (a 'U-turn' is a kind of turn).
'to detectives' becomes 'cid' (abbreviation for Criminal Investigation Department).
'l'+'u'+'cid'='LUCID'
'is' is the link.
(Other definitions for lucid that I've seen before include "Compos mentis" , "Clear-cut" , "Limpid" , "Crystal clear" , "Clearly expressed and altogether rational" .)