Working together with popular broadcast’s founder? (2,4)
I believe the answer is:
in sync
'working together' is the definition.
(working in a synchronised way)
'popular broadcast's founder?' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'broadcast's' indicates a 'sounds like' (homophone) clue (how the word might sound on the radio or TV).
'founder?' becomes 'sink' (sinking is a kind of foundering).
'sink' sounds like 'sync'.
'in'+'sync'='IN SYNC'
'with' is the link.
(Other definitions for in sync that I've seen before include "Running together correctly" , "Harmonious" , "working well together" .)