Poem, sadly unread, about love (7)
I believe the answer is:
Here is my best explanation:
'poem' is the definition.
(rondeau is a kind of poem)
'sadly unread about love' is the subsidiary indication.
'sadly' is an anagram indicator.
'about' means one lot of letters goes inside another.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'unread' anagrammed gives 'rndeau'.
'rndeau' enclosing 'o' is 'rondeau'.
(Other definitions for rondeau that I've seen before include "... and another", "Poem of ten or thirteen lines", "Short lyrical poem", "Form of poem", "set of lines", "style", "Short poem with only two rhymes and opening words used twice as refrain", "Type of poem", "Verse form" and "Style of poem".)