Key primate's farewell protects against potential release of pressure (6,5)

'potential release of pressure' is the definition.
(an escape valve might release pressure)

'key primate's farewell protects against' is the wordplay.
'key primate's' becomes 'escape' (I can't justify this - if you can you should believe this answer much more).
'farewell' becomes 'vale' (farewell in Latin).
'protects' indicates putting letters inside (outer letters protect inside letters).
'against' becomes 'v' (abbreviation for 'versus').
'vale' going around 'v' is 'valve'.
'escape'+'valve'='ESCAPE VALVE'