Vitamin in a cocktail, almost nice returned (6)
I believe the answer is:
niacin
'vitamin' is the definition.
(I know that niacin is a type of vitamin)
'in a cocktail almost nice returned' is the wordplay.
'cocktail' indicates anagramming the letters.
'almost' means to remove the last letter (almost the whole word is kept).
'returned' says the letters should be written backwards.
'nice' with its final letter removed is 'nic'.
'nic' back-to-front is 'cin'.
'in'+'a'='ina'
'ina' is an anagram of 'nia'.
'nia'+'cin'='NIACIN'
(Other definitions for niacin that I've seen before include "B vitamin which assists the nervous system" , "Vitamin, nicotinic acid" , "Vitamin, nicitinic acid" , "Another name for nicotinic acid" , "Type of vitamin" .)