Energy needed to cross river in flow (6)
I believe the answer is:
stream
'flow' is the definition.
(I know that flow can be written as stream)
'energy needed to cross river' is the wordplay.
'energy' becomes 'steam' (I've seen this before).
'needed to cross' is an insertion indicator.
'river' becomes 'r' (abbreviation for river).
'steam' enclosing 'r' is 'STREAM'.
'in' is the link.
(Other definitions for stream that I've seen before include "Rill, brook" , "'Brook, runnel (6)'" , "Quantity of water" , "Transmit music or video online" , "Steady flow; pupil group" .)