Secure the half of bitter to take outside (6)
I believe the answer is:
tether
'secure' is the definition.
(I've seen this before)
'half of bitter to take outside' is the wordplay.
'half of' means to take half the letters.
'to take' is an insertion indicator (inserted letters are taken in).
'outside' becomes 'the' (I can't justify this - if you can you should give a lot more credence to this answer).
'bitter' halved is 'ter'.
'ter' going around 'the' is 'TETHER'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for tether that I've seen before include "Range of one's endurance: tie up (horses)" , "Rope holding an animal" , "Tie to a spot" , "Rope for tying animal" , "Rope to hobble a horse" .)