More bitterns holding back river (5)
I believe the answer is:
tiber
'river' is the definition.
(River Tiber in Italy)
'more bitterns holding back' is the wordplay.
'holding' indicates the answer is hidden within the clue.
'back' says the letters should be written backwards.
'rebit' is hidden within 'more bitterns'.
'rebit' written backwards gives 'TIBER'.
(Other definitions for tiber that I've seen before include "Italian river with Ostia at its mouth" , "divides Rome" , "River entering the Mediterranean at Ostia" , "Italian banker" , "Roman river" .)