Maintain a long time is normal (7)
I believe the answer is:
average
'normal' is the definition.
('average' can be a synonym of 'normal')
'maintain a long time' is the wordplay.
'maintain' becomes 'aver' (averring is a kind of maintaining).
'a long time' becomes 'age' ('age' can be a synonym of 'long time').
'aver'+'age'='AVERAGE'
'is' acts as a link.
(Other definitions for average that I've seen before include "nothing special" , "About normal or usual" , "Mean figure?" , "Mean - everyday" , "'Medium, middling (7)'" .)