Retiring foreign character, loose old salt (7)
I believe the answer is:
oxalate
'salt' is the definition.
(oxalate is a kind of salt)
'retiring foreign character loose old' is the wordplay.
'retiring' says the letters should be written in reverse.
'foreign character' becomes 'eta' (eta is a kind of character. I am not sure about the 'foreign' bit.**).
'loose' becomes 'lax' ('lax' is associated in meaning with 'loose').
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'eta'+'lax'+'o'='etalaxo'
'etalaxo' backwards is 'OXALATE'.
Can you help me to learn more?
(Other definitions for oxalate that I've seen before include "Acid salt; a lax toe (anag.)" , "that's salty!" .)