Duck is able to get across eastern sea (5)
I believe the answer is:
ocean
'sea' is the definition.
(I know that sea can be written as ocean)
'duck is able to get across eastern' is the wordplay.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'is able' becomes 'can' (if you can do something, you are able to do it).
'to get across' is an insertion indicator.
'eastern' becomes 'e'.
'can' going around 'e' is 'cean'.
'o'+'cean'='OCEAN'
(Other definitions for ocean that I've seen before include "Indian or Arctic, say" , "eg Arctic" , "Major sea" , "It's pretty wet!" , "Large stretch of sea" .)