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?' is an anagram indicator (I've seen 'drifts' mean this).
'flotsam' with its initial letter taken off is 'lotsam'.
'lotsam' is an anagram of 'ALMOST'.
(Other definitions for almost that I've seen before include "As good as" , "practically" , "Slightly short of" , "Nearly, not quite" , "Not totally" .)