It happened in the Seventies (5)
I believe the answer is:
event
'it happened' is the definition.
(an event is something that happened)
'in the seventies' is the wordplay.
'in' says the answer is hidden in the clue.
'EVENT' is hidden within 'seventies'.
(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" .)