### Clue: Article in Le Monde describing British nuclear boffin returning without being observed (9)
### Answer: UNBEKNOWN 
 ### Explanation: Definition: without being observed
 UNE(an article in French grammar, as used in the newspaper Le Monde) **containing**(describing) B(abbrev. for "British") + **reversal of**(… returning) [ N(abbrev. for "nuclear") + WONK(one who is very clever and obsessed with a certain field) ].