The first adults only finale is going viral (8)
I believe the answer is:
trending
'going viral' is the definition.
I can't judge whether this definition defines the answer.
'the first adults only finale' is the wordplay.
'the first' means one lot of letters go next to another.
'adults only' becomes 'tr' (I can't explain this - if you can you should give a lot more credence to this answer).
'finale' becomes 'ending' (I've seen this in another clue).
'tr' put next to 'ending' is 'TRENDING'.
'is' acts as a link.
Can you help me to learn more?
(Another definition for trending that I've seen is " Becoming popular".)