This is a charade combined with a homophone - multiple parts are joined together, with one part being a word that sounds like another word.
Take the final letter from 'boss', then find a word that sounds like 'deceived' (the phrase 'on Radio 4' indicates the sound-alike). Combine these parts together.
Answer: STRICT | Take S (the last letter of 'bosS') and add TRICT, which sounds like 'tricked' (deceived). The phrase 'on Radio 4' signals the homophone.