We emptied reservoirs, supporting river authorities (6)
I believe the answer is:
powers
'authorities' is the definition.
(I've seen this in another clue)
'we emptied reservoirs supporting river' is the wordplay.
'emptied' suggests removing the centre.
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'river' becomes 'Po' (the river Po in Italy).
'reservoirs' with its centre taken out is 'rs'.
'we'+'rs'='wers'
'wers' after 'po' is 'POWERS'.
(Other definitions for powers that I've seen before include "The ...... that be are people in authority" , "Supplies with energy" , "Countries in terms of their world influence" .)