Aircraft that flew in regular service, tested initially — again! (2,5)
I believe the answer is:
at first
'again' is the definition.
Both the definition and answer are adverbs. Maybe there's a link between them I don't understand?
'aircraft that flew in regular service tested initially' is the wordplay.
'initially' says to take the initial letters.
The first letters of 'aircraft that flew in regular service tested' is 'AT FIRST'.
Can you help me to learn more?
(Other definitions for at first that I've seen before include "In the beginning, to start with" , "In the beginning, initially" , "To begin with" , "Originally" .)