One of vessels commandeered by Royal Marines supplies seafood (7)
I believe the answer is:
shrimps
'one of vessels commandeered' is the definition.
I can't tell whether this defines the answer.
'by royal marines supplies seafood' is the wordplay.
'by' says to put letters next to each other.
'royal marines supplies' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'seafood' becomes 'shrimp' (shrimp is a kind of seafood).
's' put after 'shrimp' is 'SHRIMPS'.
Can you help me to learn more?
(Other definitions for shrimps that I've seen before include "Seafood; puny people" , "Sea creatures" , "Small edible crustaceans" .)