Flying is not yet developed (2,3,3)
I believe the answer is:
in the air
'flying' is the definition.
(I've seen this before)
'not yet developed' is the wordplay.
I cannot really understand how this works, but
'not' could be 'nt' (synonyms) and 'nt' is found within the answer.
This explanation may well be incorrect...
'is' acts as a link.
Can you help me to learn more?
(Other definitions for in the air that I've seen before include "a possibility" , "Undecided; current" , "Prevalent in a vague form" , "Current, exerting an influence - may be floating" , "Flying around currently" .)