Lose rotten seafood (4)
I believe the answer is:
sole
'seafood' is the definition.
(sole is a kind of seafood)
'lose rotten' is the wordplay.
'rotten' indicates an anagram.
'lose' with letters rearranged gives 'SOLE'.
(Other definitions for sole that I've seen before include "Unique" , "A lone sort of fish - lose it" , "A fish; part of shoe" , "Underside of footwear" , "swimmer" .)