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 judge whether this definition defines the answer.
'as at the outset with development in egg?' is the subsidiary indication.
'at the outset' says to take the initial letters.
'with' is a charade indicator (letters next to each other).
'development' indicates an anagram.
The first letter of 'as' is 'a'.
'inegg' is an anagram of 'geing'.
(Other definitions for ageing that I've seen before include "Growing old", "Getting old", "Getting older" and "Growing older".)