Means the half of bitter sent in is weak and insipid (6)
I believe the answer is:
watery
'insipid' is the definition.
(I've seen this before)
'means the half of bitter sent in' is the wordplay.
'means' becomes 'way' (a way or means of doing something).
'the half of' indicates taking half.
'sent in' is an insertion indicator.
'bitter' cut in half is 'ter'.
'way' enclosing 'ter' is 'WATERY'.
'is weak and' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for watery that I've seen before include "(Of food) thin, tasteless" , "Aqueous" , "Overly diluted, thin" , "Weak, too diluted" , "Dilute; weak, pale" .)