### Clue: Mysterious travellers returned from occupying country (4)
### Answer: UFOS 
 ### Explanation: Definition: Mysterious travellers
 **Reversal of**(returned) OF(from/specifying something's origin or cause, as in "the result of a university education") **contained in**(occupying) US(abbrev. for the United States).
Defn: Unidentified Flying Objects.