It happened in the Seventies (5)
I believe the answer is:
event
'it happened' is the definition.
The definition suggests a singular noun which matches the answer.
'in the seventies' is the wordplay.
'in' says the answer is hidden in the clue.
'EVENT' is hidden within '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" .)