Supply seafood, holding big container (7)
I believe the answer is:
furnish
'supply' is the definition.
(I know that supply can be written as furnish)
'seafood holding big container' is the wordplay.
'seafood' becomes 'fish' (fish is a kind of seafood).
'holding' is an insertion indicator.
'big container' becomes 'urn' (I've seen this in another clue).
'fish' enclosing 'urn' is 'FURNISH'.
(Other definitions for furnish that I've seen before include "pianist's partner" , "Fit out with chairs etc." , "Equip" , "Supply with table and chairs etc" , "Provide, supply" .)