Tonic ingredient, one of five in recipe ultimately (7)
I believe the answer is:
quinine
'tonic ingredient' is the definition.
(I've seen this in another clue)
'one of five in recipe ultimately' is the wordplay.
'one of five' becomes 'quin' (short for quintuplet, one of five children).
'ultimately' suggests the final letters.
The final letter of 'recipe' is 'e'.
'quin'+'in'+'e'='QUININE'
(Other definitions for quinine that I've seen before include "Bitter stuff" , "Fever-reducing drug" , "Medicinal substance from cinchona bark" , "Substance used in treating malaria" , "Bitter substance from cinchona bark" .)