### Clue: Isaac's mum needs to show no sex appeal to make whoopee! (3)
### Answer: RAH 
 ### Explanation: Definition: whoopee
 "Sarah"(biblical wife of Abraham and mother of Isaac) **minus**(needs to show no) "SA"(abbrev. for "sex appeal").
Answer: Another exclamation of encouragement;excitement, similar to "whoopee!". And of course, "making whoopee" is slang for making love, which makes the clue's surface a deliberate self-contradiction!