Board not used for marine creatures (8)
I believe the answer is:
plankton
'marine creatures' is the definition.
'board not used' is the wordplay.
'board' becomes 'plank' (synonyms).
'used' indicates an anagram.
'not' is an anagram of 'ton'.
'plank'+'ton'='PLANKTON'
'for' is the link.
(Other definitions for plankton that I've seen before include "Diminutive drifter" , "Drifting marine organisms" , "Microscopic organisms in seawater" , "Drifting ocean organisms" , "Drifters in the sea" .)