Clue: Ashdown's May forfeit at end of vote: a myth? Not exactly, at last (3,2,3)
Answer: EAT MY HAT
Explanation: Definition: Ashdown's May forfeit
[VOT]E + (A MYTH)* + AT. Reference to Paddy Ashdown's promise that he would eat his hat if the exit polls at the UK General Election last May turned out be correct. They were, and he did (sort of).