Let off steam in the seventies (4)
I believe the answer is:
vent
'let off steam' is the definition.
(both can mean to release anger)
'in the seventies' is the wordplay.
'in' says the answer is hidden in the clue.
'VENT' is hidden within 'seventies'.
(Other definitions for vent that I've seen before include "Aperture to allow an escape" , "Opening of a volcano" , "Outlet; slit in jacket" , "Airhole" , "Opening releasing gas" .)