### Clue: Highly doubtful son at university acted as model! (8)
### Answer: SUPPOSED 
 ### Explanation: Definition: Highly doubtful
 S(abbrev. for "son") + UP(at/attending university) + POSED(acted as a model, for a painting or photograph, say).
Defn: …/assumed or believed to be true on flimsy evidence, as in "the supposed existence of fairies".