Unfussy consumers of old French wine upset going through customs (9)
I believe the answer is:
omnivores
'unfussy consumers' is the definition.
I can't tell whether this defines the answer.
'old french wine upset going through customs' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'french wine' becomes 'vin' ('wine' in French).
'upset' shows that the letters should be reversed in order.
'going through' indicates putting letters inside.
'customs' becomes 'mores' (as in social mores).
'vin' back-to-front is 'niv'.
'niv' inserted into 'mores' is 'mnivores'.
'o'+'mnivores'='OMNIVORES'
'of' acts as a link.
Can you help me to learn more?
(Another definition for omnivores that I've seen is " They eat anything".)