Large turn by detectives - that's clear (5)
I believe the answer is:
lucid
'clear' is the definition.
(similar in meaning)
'large turn by detectives' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'turn' becomes 'U' (a 'U-turn' is a kind of turn).
'by' is a charade indicator (letters next to each other).
'detectives' becomes 'cid' (abbreviation for Criminal Investigation Department).
'l'+'u'+'cid'='LUCID'
'that's' is the link.
(Other definitions for lucid that I've seen before include "Clear, easy to understand" , "Clearly expressed and altogether rational" , "Bright; sane" , "Clear, sane" , "Clear, intelligible" .)