### Clue: Private part of going on a diet (5)
### Answer: GONAD 
 ### Explanation: Definition: Private part
  **Hidden in**(of, or part of, if one allows "part" to double up) goinG ON A Diet. 
Answer and defn:  The ovary for a female and the testis for the male.  Strictly speaking "privates" or "private part" would only refer to genitals that are external, ie. can be seen, so gonad doesn't quite fit, though "testis"/"testicle" has been used as a substitute for the visible scrotum.