Regularly on cue, rain in Pacific maybe (5)
I believe the answer is:
ocean
'pacific maybe' is the definition.
(I know that Pacific Ocean is a ocean)
'regularly on cue rain' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'on'+'cue'+'rain'='oncuerain'
The alternate letters of 'oncuerain' are 'OCEAN'.
'in' acts as a link.
(Other definitions for ocean that I've seen before include "Extensive sea area" , "Main" , "Vast quantity of liquid" , "Large body of salt water" , "Major sea" .)