Assert length of life to be the normal run (7)
I believe the answer is:
average
'the normal run' is the definition.
'average' can be an answer for 'run' (I've seen this in another clue). I am not certain of the 'the normal' bit.
'assert length of life' is the wordplay.
'assert' becomes 'aver' (synonyms).
'length of life' becomes 'age' (I've seen this before).
'aver'+'age'='AVERAGE'
'to be' acts as a link.
Can you help me to learn more?
(Other definitions for average that I've seen before include "Run of the mill" , "Medial" , "Mean - typical" , "Arithmetical mean" , "Mean, so-so" .)