Help Rita briefly after mixing potion (7)
I believe the answer is:
philtre
'potion' is the definition.
(I know that philtre is a type of potion)
'help rita briefly after mixing' is the wordplay.
'briefly' means to remove the last letter.
'after mixing' indicates anagramming the letters.
'help'+'rita'='helprita'
'helprita' with its final letter taken off is 'helprit'.
'helprit' anagrammed gives 'PHILTRE'.
(Other definitions for philtre that I've seen before include "Exciting drink" , "Magic love potion" , "It makes you love" , "Love-potion" .)