Bad habits formed when half of 1A succeeded taking drug; ... (5)
I believe the answer is:
vices
'bad habits' is the definition.
(I've seen this before)
'1a succeeded taking drug' is the wordplay.
'va' becomes 'vic' (I can't justify this - if you can you should believe this answer much more).
'succeeded' becomes 's' (genealogical abbreviation).
'taking' means one lot of letters goes inside another.
'drug' becomes 'e' (e is another name for ecstasy).
'vic'+'s'='vics'
'vics' enclosing 'e' is 'VICES'.
'formed when half of' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for vices that I've seen before include "Shortcomings" , "Moral weaknesses and holding devices" , "Moral faults or bad habits" , "Sins or depravities" .)