Marine creatures, a great many below section of deck (8)
I believe the answer is:
plankton
'marine creatures' is the definition.
'a great many below section of deck' is the wordplay.
'a great many' becomes 'ton' (as in a ton of something).
'below' means one lot of letters go next to another.
'section of deck' becomes 'plank' (a plank is a part of the deck on a ship).
'ton' put after 'plank' is 'PLANKTON'.
(Other definitions for plankton that I've seen before include "Marine organism" , "Food source for aquatic creatures" , "Drifting aquatic organisms" , "Tiny organisms drifting in the sea" , "Tiny creatures" .)