Declare duration of life ordinary (7)
I believe the answer is:
average
'ordinary' is the definition.
(I know that ordinary can be written as average)
'declare duration of life' is the wordplay.
'declare' becomes 'aver' (averring is a kind of declaring).
'duration of life' becomes 'age' (I can't explain this - if you can you should give a lot more credence to this answer).
'aver'+'age'='AVERAGE'
Can you help me to learn more?
(Other definitions for average that I've seen before include "Mean, so-so" , "Mean - ordinary" , "Mediocre, commonplace" , "Moderate" , "'Medium, middling (7)'" .)