Lady's with Fancy Dan, hiding in bush? They're lovers (4,3,7)
I believe the answer is:
hero and leander
'they're lovers' is the definition.
(lovers in Greek myth)
'lady's with fancy dan hiding in bush?' is the wordplay.
'lady's' becomes 'her' (belonging to a lady).
'with' is a charade indicator (letters next to each other).
'fancy' indicates an anagram.
'hiding in' means one lot of letters goes inside another.
'bush?' becomes 'oleander' (oleander is a type of bush).
'dan' with letters rearranged gives 'and'.
'and' placed within 'oleander' is 'oandleander'.
'her'+'oandleander'='HERO AND LEANDER'
(Another definition for hero and leander that I've seen is " Classical myth".)