Some careless officer found upside down in a ditch (5)
I believe the answer is:
fosse
'a ditch' is the definition.
(fosse is a kind of ditch)
'some careless officer found upside down' is the wordplay.
'found' indicates the answer is hidden within the clue.
'upside down' is a reversal indicator.
'essof' can be found hidden inside 'some careless officer'.
'essof' reversed gives 'FOSSE'.
'in' acts as a link.
(Other definitions for fosse that I've seen before include "Fortification ditch" , "Long narrow ditch" , "Long, narrow trench or excavation" , "-- Way, Roman road" , "Long trench or ditch" .)