Confusing loner, it's vitamin A (7)
I believe the answer is:
retinol
'vitamin a' is the definition.
(vitamin A refers to retinol)
'confusing loner it's' is the wordplay.
'confusing' is an anagram indicator.
'loner'+'it'='lonerit'
'lonerit' is an anagram of 'RETINOL'.
(Other definitions for retinol that I've seen before include "vitamin essential for good eyesight" , "certain vitamin" .)