Christopher adds it to a cocktail (3)
I believe the answer is:
kit
'christopher' is the definition.
(short for Christopher)
'it to a cocktail' is the wordplay.
'to a' is a charade indicator (letters next to each other) (I've seen this in other clues).
'cocktail' becomes 'K' (the end letter or tail of the word 'cocktail').
'it' put after 'k' is 'KIT'.
'adds' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for kit that I've seen before include "tackle" , "Set of parts sold to be assembled" , "Specialised clothing or set of parts for assembly" , "In strip" , "Supplies" .)