I go by mainly old Byzantine life lessons (7)
I believe the answer is:
biology
'life lessons' is the definition.
Although both the answer and 'lessons' are nouns, they are not in the same form of the noun.
'i go by mainly old byzantine' is the wordplay.
'mainly' means to remove the last letter.
'byzantine' indicates anagramming the letters.
'old' with its last letter taken away is 'ol'.
'i'+'go'+'by'+'ol'='igobyol'
'igobyol' with letters rearranged gives 'BIOLOGY'.
Can you help me to learn more?
(Other definitions for biology that I've seen before include "Study of living things" , "The science that studies living organisms" , "Study of living organisms" , "Science of living things" .)