### Clue: Weak device for transporting policeman finally engaged in one detective story (9)
### Answer: WHODUNNIT 
 ### Explanation: Definition: detective story
 W(abbrev. for "weak" in quantum physics, in relation to the weak interaction between subatomic particles)+ HOD(a device for transporting bricks and other building materials) + [ **last letter of**(… finally) "policeman" **contained in**(engaged in) UNIT(a singular quantity/one) ].**
Whodunn presented the TV mystery game show?  You Know Who, of course.