Oasis or Nirvana dropping Ecstasy (5)
I believe the answer is:
haven
'oasis' is the definition.
('haven' can be a synonym of 'oasis')
'nirvana dropping ecstasy' is the wordplay.
'nirvana' becomes 'heaven' ('heaven' can be a synonym of 'nirvana').
'dropping' suggests deleting specific letters.
'ecstasy' becomes 'e' (informal term for the drug).
'heaven' with 'e' removed is 'HAVEN'.
'or' acts as a link.
(Other definitions for haven that I've seen before include "Place of shelter" , "Sheltered port" , "Harbour, small port" , "Safe refuge" , "Shelter (for ships)" .)