Boat lower when stern disappears (4)
I believe the answer is:
scow
'boat' is the definition.
(scow is a kind of boat)
'lower when stern disappears' is the wordplay.
I cannot quite understand how this works, but
'lower' could be 'cow' and 'cow' is found within the answer.
A single letter 's' remains which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for scow that I've seen before include "Maybe lighter" , "Unpowered freight barge" , "Low boat" , "Flat-bottomed boat for cows" , "Wide-beamed dinghy" .)