Clue: Pest breaking the law in France — "why?", they ask (5,3)
Answer: WHEAT FLY
Explanation: Definition: Pest
F[rance] in (THE LAW)* + Y (homophone of "why") – the "in" seems the wrong way round, but you can just about read it as "..(THE LAW)* [and] in [we have to put] F"