The necessary spot of sweetener in tonic (4)
I believe the answer is:
dosh
'the necessary' is the definition.
(both can mean money)
'spot of sweetener in tonic' is the wordplay.
'spot of' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'tonic' becomes 'doh' (musical tonic).
The initial letter of 'sweetener' is 's'.
's' going into 'doh' is 'DOSH'.
(Other definitions for dosh that I've seen before include "Brass" , "currency" , "cash" , "Contents of purse" , "Moolah (slang)" .)