Egg, in a different way, is growing old (6)
I believe the answer is:
ageing
'is growing old' is the definition.
(I've seen this before)
'egg in a different way' is the wordplay.
'different way' indicates anagramming the letters.
'egg'+'in'+'a'='eggina'
'eggina' is an anagram of 'AGEING'.
(Other definitions for ageing that I've seen before include "Elderly" , "getting more advanced?" , "rather old" , "Growing older" , "Growing old" .)