Feed egg to one about to enter a water body (3,4)
I believe the answer is:
oil cake
'feed' is the definition.
(oil cake is a kind of feed)
'egg to one about to enter a water body' is the wordplay.
I cannot quite see how this works, but
'egg' could be 'o' (an 'o' resembles an egg) and 'o' is present in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
'about' could be 'ca' (short for circa) and 'ca' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for oil cake that I've seen is " cattle feed".)