Bird entertained by boy in garden, holder of massive flower (5,3)
I believe the answer is:
aswan dam
'holder of massive flower' is the definition.
(holds back the River Aswan, a thing which 'flows')
'bird entertained by boy in garden' is the wordplay.
'bird' becomes 'swan' (swan is a kind of bird).
'entertained by' indicates putting letters inside (I've seen this in other clues).
'boy in garden' becomes 'adam' (Biblical reference).
'swan' inserted into 'adam' is 'ASWAN DAM'.
(Other definitions for aswan dam that I've seen before include "massive barrier" , "Egyptian water barrier" , "Egypt's lake district" , "Nile construction of 1902" , "Huge structure in Africa" .)