Essence is put in gin & tonic (4)
I believe the answer is:
gist
'essence' is the definition.
(I know that essence can be written as gist)
'is put in gin tonic' is the wordplay.
'put in' means one lot of letters goes inside another.
'gin tonic' becomes 'gt' (this might be a standard abbreviation I've not previously seen).
'is' put within 'gt' is 'GIST'.
Can you help me to learn more?
(Other definitions for gist that I've seen before include "Substance or general idea" , "Central meaning or theme" , "Real point or substance" , "Main point of the matter" , "Essential idea" .)