Romance (4,6)
'romance' is the definition.
(synonyms)
This is all the clue.
(Other definitions for love affair that I've seen before include "Romance" , "A (short?) romantic relationship" .)
(Other definitions for modern that I've seen before include "'Contemporary, up-to-the-minute (6)'" , "Up to date, of present times" , "'Contemporary, up-to-date (6)'" , "Novel" , "See 11" .)