Clue: Backtrack twice and workers lit up by party leader? I don't believe you! (4,4,5,2,4)
Answer: LIAR LIAR PANTS ON FIRE
Explanation: Definition: I don't believe you!
A charade of LIAR LIAR, a reversal ('back'-) 'twice' of RAIL (-'track') plus P ('**P**arty leader') plus ANTS ('workers') plus ON ~~FILE~~ *FIRE *('lit up'), with 'by' suggesting the order of the particles.