Seafood one’s taken from ships crossing border (7)
I believe the answer is:
shrimps
'seafood' is the definition.
(shrimp is a type of seafood)
'one's taken from ships crossing border' is the wordplay.
'one' becomes 'i' (Roman numeral).
'taken from' suggests deleting specific letters.
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'border' becomes 'rim' (both can mean an edge).
'ships' with 'i' taken out is 'shps'.
'shps' enclosing 'rim' is 'SHRIMPS'.
(Other definitions for shrimps that I've seen before include "Seafood; puny people" , "Small edible crustaceans" , "Sea creatures" .)