Ten returned without an unknown quantity of French compound (5)
I believe the answer is:
oxide
'compound' is the definition.
(I know that oxide is a type of compound)
'ten returned without an unknown quantity of french' is the wordplay.
'ten' becomes 'IO' (IO resembles 10).
'returned' is a reversal indicator.
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'an unknown quantity' becomes 'X' (algebra).
'of french' becomes 'de' ('of' in French).
'io' back-to-front is 'oi'.
'oi' placed around 'x' is 'oxi'.
'oxi'+'de'='OXIDE'
(Other definitions for oxide that I've seen before include "Compound of oxygen and one other element" , "Chemical compound" , "Compound such as rust" , "Rust, for instance" , "Rust is iron -" .)