Ingredient of tonic, a must-have from the East (5)
I believe the answer is:
sumac
'ingredient' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'tonic a must-have from the east' is the wordplay.
'have' indicates the answer is hidden within the clue.
'from the east' is a reversal indicator.
'camus' is hidden in the letters of 'tonic a must'.
'camus' reversed gives 'SUMAC'.
'of' acts as a link.
(Other definitions for sumac that I've seen before include "Rhus tree" , "Rudely brief" , "Shrub or small tree - Camus (anag)" , "Novelist" , "Garden shrub" .)