Don't forget viral phenomenon about old teacher retiring (8)
I believe the answer is:
memorise
'don't forget viral phenomenon' is the definition.
I can't judge whether this definition defines the answer.
'about old teacher retiring' is the wordplay.
'about' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'teacher' becomes 'ris' (Researcher in Schools).
'retiring' becomes 'meme' (I can't justify this - if you can you should give a lot more credence to this answer).
'o'+'ris'='oris'
'oris' inserted into 'meme' is 'MEMORISE'.
Can you help me to learn more?
(Other definitions for memorise that I've seen before include "Commit information to the mind" , "Learn by heart" .)