Success, declining a drink with tonic (3)
I believe the answer is:
gin
'drink with tonic' is the definition.
(drink often mixed with tonic)
'success declining a' is the wordplay.
'success' becomes 'gain'.
'declining' indicates named letters should be taken away.
'gain' with 'a' removed is 'GIN'.
(Other definitions for gin that I've seen before include "Animal trap or alcoholic drink" , "Spirit; cotton machine" , "Juniper-flavoured drink" , "Liquor flavoured with juniper berries" , "Trap; mother's ruin" .)