Told the authorities about Liberal speech, informally (5)
I believe the answer is:
slang
'speech informally' is the definition.
'told the authorities about liberal' is the wordplay.
'told the authorities' becomes 'sang' ('sing' can mean to act as an informant).
'about' is an insertion indicator.
'liberal' becomes 'l' (abbreviation).
'sang' placed around 'l' is 'SLANG'.
(Other definitions for slang that I've seen before include "Colloquial language" , "Unconventional language" , "abuse" , "Informal speech" , "Any class's jargon" .)