Exactly identify amount of fluid collecting in river (8)
I believe the answer is:
pinpoint
'exactly identify' is the definition.
(to pinpoint is to identify or track down precisely)
'amount of fluid collecting in river' is the wordplay.
'amount of fluid' becomes 'pint'.
'collecting' indicates putting letters inside.
'river' becomes 'Po River' (Italian river).
'in'+'po'='inpo'
'pint' enclosing 'inpo' is 'PINPOINT'.
(Other definitions for pinpoint that I've seen before include "something sharp" , "Tiny dot; absolutely precise" , "Put one's finger on" , "Define precisely" , "Locate or identify with precision" .)