As at the outset with development in egg? (6)
I believe the answer is:
Here is my best explanation:
'as at the outset with development in egg?' is the definition.
I can't tell whether this defines the answer.
'as at the outset with development in egg?' is the wordplay.
'at the outset' indicates taking the first letters.
'with' is a charade indicator (letters next to each other).
'development' indicates anagramming the letters.
The first letter of 'as' is 'a'.
'inegg' with letters rearranged gives 'geing'.
(Other definitions for ageing that I've seen before include "no longer fresh", "rather old", "Growing older", "Elderly", "process of development", "Getting old", "Getting older", "Growing old", "Declining" and "getting more advanced?".)