Detectives turned into strange speech (7)
I believe the answer is:
diction
'speech' is the definition.
(diction is someone's way of speaking)
'detectives turned into strange' is the wordplay.
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'turned' shows that the letters should be reversed in order.
'strange' is an anagram indicator.
'cid' in reverse letter order is 'dic'.
'into' is an anagram of 'tion'.
'dic'+'tion'='DICTION'
(Other definitions for diction that I've seen before include "Manner of enunciation" , "Style of enunciation" , "words for speech" , "Way of speaking or pronouncing" , "Choice of words" .)