Essential spirit finally found in small bottle (5)
I believe the answer is:
vital
'essential' is the definition.
(similar in meaning)
'spirit finally found in small bottle' is the wordplay.
'finally' suggests the final letters.
'found in' means one lot of letters goes inside another.
'small bottle' becomes 'vial' (I've seen this in another clue).
The final letter of 'spirit' is 't'.
't' inserted into 'vial' is 'VITAL'.
(Other definitions for vital that I've seen before include "Essential; lively" , "Urgently needed or absolutely necessary" , "Imperative" , "Of key importance" , "Of the greatest importance" .)