What’s essential — some speculative returns (5)
I believe the answer is:
vital
'what's essential' is the definition.
(similar in meaning)
'some speculative returns' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'returns' says the letters should be written in reverse.
'lativ' can be found hidden inside 'speculative'.
'lativ' back-to-front is 'VITAL'.
(Other definitions for vital that I've seen before include "Most important, like certain body organs" , "Extremely important, essential to success" , "alive" , "Critical; life-sustaining" , "Imperative" .)