It's crucial to put the last of it in a bottle (5)
I believe the answer is:
vital
'it's crucial' is the definition.
(I've seen this before)
'the last of it in a bottle' is the wordplay.
'the last of' indicates one should take the final letters.
'in' indicates putting letters inside.
'a bottle' becomes 'vial' (vial is a kind of bottle).
The last letter of 'it' is 't'.
't' placed into 'vial' is 'VITAL'.
'to put' acts as a link.
(Other definitions for vital that I've seen before include "Are such statistics" , "Crucially important, essential" , "Highly important" , "Essential to life, indispensable" , "Most important, like certain body organs" .)