Clue: Oddly ingenious PM's working to engage crime-fighting assistant (5,8)
Answer: HEATH ROBINSON
Explanation: Oddly ingenious PM's working to engage crime-fighting assistant (5,8)
HEATH'S ON ("PM's working") "to engae" ROBIN (Batman's "crime-fighting assistant") 
W Heath Robinson famously created overcomplicated designs for machines to solve simple problems.