Depression following popular Channel Islands event (8)
I believe the answer is:
incident
'event' is the definition.
(incident is a kind of event)
'depression following popular channel islands' is the wordplay.
'depression' becomes 'dent' (dent is a kind of depression).
'following' is a charade indicator (letters next to each other).
'popular' becomes 'in' ('in' can mean trendy).
'channel islands' becomes 'CI'.
'in'+'ci'='inci'
'dent' after 'inci' is 'INCIDENT'.
(Other definitions for incident that I've seen before include "Event, occurence" , "Public disturbance" , "Distinct event" , "hostile clash" , "Minor event or occurrence" .)