Sweetheart from Vienna let off (9)
I believe the answer is:
valentine
'sweetheart' is the definition.
(I know that valentine is a type of sweetheart)
'vienna let off' is the wordplay.
'off' is an anagram indicator.
'vienna'+'let'='viennalet'
'viennalet' anagrammed gives 'VALENTINE'.
'from' is the link.
(Other definitions for valentine that I've seen before include "Anonymous love token from an admirer" , "Saint regarded as the patron of lovers" , "Potential lover" , "One of Shak.'s Two Gentlemen" , "Card to sweetheart" .)