### Clue: across's ultimate invention that's timeless, followed by a complete devotee (10)
### Answer: AFICIONADO 
 ### Explanation: Definition: devotee
 **Last letter of**(ultimate) answer to 10across + "fiction"(invention;fabrication) **minus**(that's …less) "t"(abbrev. for "time") **plus**(followed by) A + DO(to complete an action or deed).