It happened in the Seventies (5)
'it happened' is the definition.
The definition suggests a singular noun which matches the answer.
'in the seventies' is the wordplay.
'in' indicates a hidden word.
'EVENT' is hidden in the letters of 'seventies'.
Can you help me to learn more?
(Other definitions for event that I've seen before include "Something going on", "Incident - final outcome", "case", "Something that happens - part of a programme?", "special occasion".)