Escape from extremely grave swell (3,3)
I believe the answer is:
get off
'escape' is the definition.
('get off' can be a synonym of 'escape')
'extremely grave swell' is the wordplay.
'extremely' means to remove the middle letters.
'swell' becomes 'toff' (wealthy or high-class person).
'grave' with its middle taken out is 'ge'.
'ge'+'toff'='GET OFF'
'from' acts as a link.
(Other definitions for get off that I've seen before include "Alight" , "Disembark" , "Escape all punishment" , "Not be punished" , "Leave train or dismount" .)