It's essential that a quarter-pint goes in a small bottle (5)
I believe the answer is:
vital
'it's essential' is the definition.
(similar in meaning)
'a quarter-pint goes in a small bottle' is the wordplay.
'a quarter pint goes' becomes 't' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'a small bottle' becomes 'vial' (I've seen this before).
't' placed within 'vial' is 'VITAL'.
'that' is the link.
Can you help me to learn more?
(Other definitions for vital that I've seen before include "Essential to life, indispensable" , "Spirited" , "Essential for life; key" , "Absolutely necessary or life-giving" , "Urgently needed or absolutely necessary" .)