Maintain period is normal (7)
I believe the answer is:
average
'is normal' is the definition.
(synonyms)
'maintain period' is the wordplay.
'maintain' becomes 'aver' (averring is a kind of maintaining).
'period' becomes 'age' (age is a kind of period).
'aver'+'age'='AVERAGE'
(Other definitions for average that I've seen before include "par" , "'Medium, middling (7)'" , "Medium; normal" , "Normal; mean" , "Mediocre - medium" .)