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", "Form of poem", "Style of poem", "Short poem with only two rhymes and opening words used twice as refrain", "Poem of ten or thirteen lines", "Verse form", "Short lyrical poem", "Type of poem" and "Musical composition; poem of 13/15 lines".)