### Clue: Twice one's substituted for United? That's realistic (6)
### Answer: DOABLE 
 ### Explanation: Definition: realistic
 "double"(twice/twofold) **with** "a"(in grammar, an article denoting one in quantity) **replacing**(…'s substituted for) "u"(abbrev. for "united").
Defn: …/achievable.