No warships regularly retreat (5)
'retreat' is the definition.
(I've seen this before)
'no warships regularly' is the wordplay.
'no' becomes 'o' (resembles 0 - 'no' can mean 'zero').
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'warships' are 'asis'.
(Other definitions for oasis that I've seen before include "Place of refreshment", "Fertile spot in desert", "A place of safety or sanctuary", "Desert pool", "quiet spot".)