Sleep in capsized vessel (3)
I believe the answer is:
nap
'sleep' is the definition.
(a nap is a short sleep)
'capsized vessel' is the wordplay.
'capsized' says the letters should be written backwards (in a down clue, the word turns upside down).
'vessel' becomes 'pan' (vessel or container for cooking).
'pan' in reverse letter order is 'NAP'.
'in' acts as a link.
(Other definitions for nap that I've seen before include "A brief snooze" , "One likely to win" , "Quick sleep; raised pile" , "Pile of material" , "Kip" .)