Flotsam in ocean contains a type of acid (5)
I believe the answer is:
amino
'a type of acid' is the definition.
'flotsam in ocean contains' is the wordplay.
'contains' says the answer is hidden in the clue.
'AMINO' is hidden within 'flotsam in ocean'.
(Other definitions for amino that I've seen before include "Minor insult; slim" , "Acid found in proteins" , "sort of acid?" , "- - acids" , "(Protein constituent) acid" .)