Water round Cape with remains regularly seen (5)
I believe the answer is:
ocean
'water' is the definition.
(oceans are bodies of water)
'round cape with remains regularly seen' is the wordplay.
'round' becomes 'o' (o is a circular letter).
'cape' becomes 'c' (abbreviation used on maps).
'with' means one lot of letters go next to another.
'regularly seen' means one should take alternating letters.
The alternating letters of 'remains' are 'ean'.
'o'+'c'+'ean'='OCEAN'
(Other definitions for ocean that I've seen before include "Large expanse of water" , "Indian perhaps" , "It's pretty wet!" , "Extensive sea area" , "One of the Seven Seas" .)