Seafood board not put up? (8)
I believe the answer is:
plankton
'seafood' is the definition.
(I've seen this before)
'board not put up?' is the wordplay.
'board' becomes 'plank' (synonyms).
'put up?' shows that the letters should be reversed in order (I've seen 'put up' mean this).
'not' written backwards gives 'ton'.
'plank'+'ton'='PLANKTON'
(Other definitions for plankton that I've seen before include "drifting masses" , "Microscopic organisms that float on the surface of water" , "Food source for aquatic creatures" , "Whale's staple diet" , "Minute fish food" .)