Dancing girl lay behind house, the object of desire (4,5)
I believe the answer is:
Here is my best explanation:
'the object of desire' is the definition.
I can't judge whether this defines the answer.
'dancing girl lay behind house' is the wordplay.
'dancing' is an anagram indicator.
'behind' means one lot of letters go next to another.
'house' becomes 'ho' (abbreviation for house).
'girllay' with letters rearranged gives 'lygrail'.
'lygrail' put after 'ho' is 'HOLY GRAIL'.
(Other definitions for holy grail that I've seen before include "Cup that was much sought after?", "object of quest", "Go, Hillary! (anag) -- chalice used by Jesus at the Last Supper", "A cherished ambition", "Cherished ambition", "Something eagerly sought", "Object of search", "Christ's cup at the Last Supper, sought by medieval knights", "Chalice used by Christ at the Last Supper" and "In legend, the cup used by Christ at the Last Supper".)