Maybe many decades are viewed in retrospect (3)
I believe the answer is:
era
'retrospect' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'maybe many decades are viewed' is the wordplay.
'are' anagrams to 'ERA'.
But, I am not sure how the anagram is indicated.
'in' is the link.
Can you help me to learn more?
(Other definitions for era that I've seen before include "Epoch, period" , "Geological period" , "Are involved in time span" , "Distinctive period of time" , "Geological age" .)