### Clue: Rees-Mogg finally abandoning common sense in support of fool's unjustified claim (10)
### Answer: ASSUMPTION 
 ### Explanation: Definition: unjustified claim
 **Last letter of**(… finally) "Rees-Mogg" **deleted from**(abandoning) "gumption"(perhaps even more than common sense/shrewdness) **placed below**(in support of, in a down clue) ASS(a fool).