### Clue: Aircraft that flew in regular service, tested initially — again! (2,5)
### Answer: AT FIRST 
 ### Explanation: Definition: again!
definition: "again" referring to using "initially" as the definition, as well as in the wordplay 
initial letters from A-ircraft + T-hat + F-lew + I-n + R-egular + S-ervice + T-ested