Learn viral video contains PM scratching head (8)
I believe the answer is:
memorise
'learn' is the definition.
('learn'->'memorize' is in my internal thesaurus and 'ize'->'ise')
'viral video contains pm scratching head' is the wordplay.
'viral video' becomes 'meme' (as in an internet meme).
'contains' is an insertion indicator.
'pm' becomes 'boris' (Boris Johnson, prime minister).
'scratching head' means to remove the first letter.
'boris' with its initial letter taken off is 'oris'.
'meme' going around 'oris' is 'MEMORISE'.
(Other definitions for memorise that I've seen before include "Learn by heart" , "Commit information to the mind" .)