Regularly resell seafood (3)
I believe the answer is:
eel
'seafood' is the definition.
(eel is a kind of seafood)
'regularly resell' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'resell' are 'EEL'.
(Other definitions for eel that I've seen before include "Slithery fish" , "Teleost fish" , "Slippery type" , "Slimy fish" , "Large elver" .)