It's essential to put half of it in a little bottle (5)
I believe the answer is:
vital
'it's essential' is the definition.
(similar in meaning)
'half of it in a little bottle' is the wordplay.
'half of' means to take half the letters.
'in' means one lot of letters goes inside another.
'a little bottle' becomes 'vial' (vial is a kind of bottle. I am not sure about the 'little' bit.).
'it' cut in half is 't'.
't' going into 'vial' is 'VITAL'.
'to put' is the link.
Can you help me to learn more?
(Other definitions for vital that I've seen before include "Desperately important" , "Characteristic of life" , "Essential for life; key" , "Extremely important, essential to success" , "Essential to life, indispensable" .)