Necessary quarter-pint in a bottle (5)
I believe the answer is:
vital
'necessary' is the definition.
(I've seen this in another clue)
'quarter-pint in a bottle' is the wordplay.
'quarter pint' becomes 't' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'a bottle' becomes 'vial' (vial is a kind of bottle).
't' inserted into 'vial' is 'VITAL'.
Can you help me to learn more?
(Other definitions for vital that I've seen before include "Crucially important, essential" , "Urgently needed or absolutely necessary" , "Of the greatest importance" , "Critical; life-sustaining" , "Most important, like certain body organs" .)