Horned beast losing oxygen and energy in river (5)
I believe the answer is:
rhine
'river' is the definition.
(Rhine is an example)
'horned beast losing oxygen and energy' is the wordplay.
'horned beast' becomes 'rhino' (I've seen this before).
'losing' indicates named letters should be taken away.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'and' is a charade indicator (letters next to each other).
'energy' becomes 'e' (physics symbol as in E=mc2).
'rhino' with 'o' taken out is 'rhin'.
'rhin'+'e'='RHINE'
'in' is the link.
(Other definitions for rhine that I've seen before include "River rising in Alps, with mouth at Rotterdam" , "Ring river (Wagner)" , "River at Bonn, Koln" , "fed by 8 [RHUR]" , "Bonn's river" .)