Below stairs, oddly, eat fish (7)
I believe the answer is:
sardine
'fish' is the definition.
(sardine is a kind of fish)
'below stairs oddly eat' is the wordplay.
'below' is a charade indicator (letters next to each other) (I've seen this in other clues).
'oddly' indicates alternate letters.
'eat' becomes 'dine' (dining is a kind of eating).
The alternate letters of 'stairs' are 'sar'.
'sar'+'dine' is 'SARDINE'.
(Other definitions for sardine that I've seen before include "A diner's small fish" , "Fish (often tinned)" , "FRENCH FISH" , "Young pilchard" , "Swimmer" .)