Bird entertained by boy in garden, holder of massive flower (5,3)
I believe the answer is:
Here is my best explanation:
'holder of massive flower' is the definition.
(holds back the River Aswan, a thing which 'flows')
'bird entertained by boy in garden' is the subsidiary indication.
'bird' becomes 'swan' (swan is a kind of bird).
'entertained by' means one lot of letters goes inside another (I've seen this in other clues).
'boy in garden' becomes 'adam' (Biblical reference).
'swan' inserted into 'adam' is 'aswandam'.
(Other definitions for aswan dam that I've seen before include "Nile construction of 1902" and "Egyptian water barrier".)