Flotsam not beginning to drift? Not quite! (6)
I believe the answer is:
almost
'not quite' is the definition.
(I've seen this before)
'flotsam not beginning to drift?' is the wordplay.
'not beginning' means to remove the first letter.
'to drift?' indicates an anagram (I've seen 'drifting' mean this).
'flotsam' with its initial letter removed is 'lotsam'.
'lotsam' anagrammed gives 'ALMOST'.
(Other definitions for almost that I've seen before include "As good as" , "practically" , "Slightly short of" , "Nearly, not quite" , "Not totally" .)