Ocean drifters not put back on board (8)
I believe the answer is:
plankton
'ocean drifters' is the definition.
(plankton drift through the ocean)
'not put back on board' is the wordplay.
'put back' says the letters should be written in reverse.
'on' means one lot of letters go next to another.
'board' becomes 'plank' (synonyms).
'not' in reverse letter order is 'ton'.
'ton' put after 'plank' is 'PLANKTON'.
(Other definitions for plankton that I've seen before include "Microscopic organic life that floats in sea" , "Drifting marine organisms" , "Drifters in the sea" , "food for school, possibly" , "Microscopic organisms in seawater" .)