Big cat is losing energy in Asian river (6)
I believe the answer is:
tigris
'river' is the definition.
(I know that Tigris is a river)
'big cat is losing energy in asian' is the wordplay.
'big cat' becomes 'tiger' (tiger is a kind of big cat).
'is losing' suggests deleting specific letters.
'energy' becomes 'e' (physics symbol as in E=mcs).
'in asian' becomes 'is' (I can't justify this - if you can you should believe this answer much more).
'tiger' with 'e' removed is 'tigr'.
'tigr'+'is'='TIGRIS'
Can you help me to learn more?
(Other definitions for tigris that I've seen before include "It flows through Iraq" , "Sit rig into the Asian river" , "Flower of Baghdad" , "Major tributary of the Euphrates" , "Great river flowing from Turkey through Iraq" .)