Once capsized crossing a body of water (5)
I believe the answer is:
ocean
'body of water' is the definition.
(I've seen this before)
'once capsized crossing a' is the wordplay.
'capsized' indicates anagramming the letters.
'crossing' indicates putting letters inside.
'once' is an anagram of 'ocen'.
'ocen' placed around 'a' is 'OCEAN'.
(Other definitions for ocean that I've seen before include "Very large area of sea" , "Large amount of water and" , "Large area of water" , "Indian, for example" , "Big sea" .)