Oscar, for example, having a temperature, always lost (4)
I believe the answer is:
fish
'oscar for example' is the definition.
(oscar is a type of fish)
'having a temperature always lost' is the wordplay.
'having a temperature' becomes 'feverish'.
'always' becomes 'ever' (synonyms).
'lost' is a deletion indicator.
'feverish' with 'ever' taken away is 'FISH'.
(Other definitions for fish that I've seen before include "Aquatic creature; rail-joining plate" , "Angle, troll" , "Ruff" , "Burbot or turbot, for example" , "To try to catch" .)