No warships regularly retreat (5)
I believe the answer is:
oasis
'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'.
'o'+'asis'='OASIS'
(Other definitions for oasis that I've seen before include "A place of safety or sanctuary" , "Feature of desert" , "Fertile place in a desert, a watering hole" , "beauty spot" , "quiet spot" .)