One gives inside info of Cockney three-piece on phone (7-6)
I believe the answer is:
whistle-blower
'one gives inside info' is the definition.
'cockney three-piece on phone' is the wordplay.
'cockney three-piece' becomes 'whistle' (whistle-and-suit is Cockney rhyming slang for suit).
'on' means one lot of letters go next to another.
'phone' becomes 'blower' (informal term for telephone).
'whistle'+'blower'='WHISTLE-BLOWER'
'of' is the link.
(Other definitions for whistle-blower that I've seen before include "One who informs on malpractice" , "Informer on illicit activities" , "Person with inside information making public some illicit activity" , "Teller of tales to the authorities - referee?" , "Person revealing improper activities" .)