State decline is normal (7)
I believe the answer is:
average
'is normal' is the definition.
(similar in meaning)
'state decline' is the wordplay.
'state' becomes 'aver' (both can mean to declare).
'decline' becomes 'age' (get older).
'aver'+'age'='AVERAGE'
(Other definitions for average that I've seen before include "Mediocre, commonplace" , "It may be mean" , "par" , "nothing special" , "Normal in arithmetic way" .)