Grow old under state routine (7)
I believe the answer is:
average
'routine' is the definition.
(thesaurus)
'grow old under state' is the wordplay.
'grow old' becomes 'age' ('age' can be a synonym of 'grow old').
'under' is a charade indicator (letters next to each other).
'state' becomes 'aver' (I've seen this before**).
'age' after 'aver' is 'AVERAGE'.
(Other definitions for average that I've seen before include "Mean - everyday" , "Mean figure?" , "Moderate" , "Mean - typical" , "Medium; normal" .)