Inferior to a French wine knocked back (5)
I believe the answer is:
Here is my best explanation:
'inferior' is the definition.
'a french wine knocked back' is the wordplay.
'a french' becomes 'un' ('a' in French).
'wine' becomes 'red' (red wine is a kind of wine**).
'knocked back' is a reversal indicator.
'red' backwards is 'der'.
'to' is the link.
(Other definitions for under that I've seen before include "Below or subordinate to", "Beneath or subordinate to", "In a state of unconsciousness", "Managed by", "Governed by", "Subordinate to; planted with", "Lower than", "Not above", "Below, anaesthetised" and "Anaesthetised".)