Leading pair of trolls cease to be popular online (5)
I believe the answer is:
trend
'be popular online' is the definition.
(I've seen this in another clue)
'leading pair of trolls cease' is the wordplay.
'leading' says to put letters next to each other.
'pair of trolls' becomes 'tr' (I can't justify this - if you can you should give a lot more credence to this answer).
'cease' becomes 'end' (synonyms).
'tr' next to 'end' is 'TREND'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for trend that I've seen before include "General line of orientation" , "Drift; vogue" , "Direction, tendency" , "General tendency" , "Med. port, taken by Crusaders" .)