### Clue: Letter about returning to Jenny oddly lacking insight (6)
### Answer: ACUMEN 
 ### Explanation: Definition: insight
 **Reversal of**(… returning, in a down clue) [MU(letter in the Greek alphabet) + CA(abbrev. for "circa"/about when refering to a year, or a period of years)] **plus**(to) "Jenny" **minus its 1st, 3rd and 5th letters** (oddly lacking).