Ingredient in a drink not stirred, I see (5)
I believe the answer is:
tonic
'ingredient in a drink' is the definition.
'tonic' can be an answer for 'drink' (tonic is a kind of drink). I am not certain of the 'ingredient in a' bit.
'not stirred i see' is the wordplay.
'stirred' is an anagram indicator.
'see' becomes 'C' (the word for the letter, according to Chambers).
'not' with letters rearranged gives 'ton'.
'ton'+'i'+'c'='TONIC'
Can you help me to learn more?
(Other definitions for tonic that I've seen before include "do for musicians" , "It goes with gin" , "something nice and fresh" , "something medicinal" , "Stimulus" .)