Dead old fish covered in batter, oddly (8)
I believe the answer is:
obsolete
'dead' is the definition.
('obsolete' can be similar in meaning to 'dead')
'old fish covered in batter oddly' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'fish' becomes 'sole' (sole is a kind of fish**).
'covered in' means one lot of letters goes inside another.
'oddly' means one should take alternating letters.
The alternate letters of 'batter' are 'bte'.
'sole' placed inside 'bte' is 'bsolete'.
'o'+'bsolete'='OBSOLETE'
(Other definitions for obsolete that I've seen before include "Boots eel away, being out of date" , "Outdated" , "Discontinued" , "No longer produced or used" , "No longer useful" .)