Troubadour's topic (4)
I believe the answer is:
love
'troubadour's topic' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for love that I've seen before include "0 (tennis)" , "Passionate desire" , "It's nothing in tennis" , "Vole (anag.)" , "Zero score, dear" .)