Assert a long time the level regarded as usual (7)
I believe the answer is:
average
'usual' is the definition.
(I've seen this before)
'assert a long time' is the wordplay.
'assert' becomes 'aver' (synonyms).
'a long time' becomes 'age' (an age is a period of time).
'aver'+'age'='AVERAGE'
'the level regarded as' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for average that I've seen before include "par" , "Mean - ordinary" , "building society customer loses capital over time" , "Mean; standard" , "It's common" .)