It is over where boxers appear to be running out of energy (6)
I believe the answer is:
tiring
'boxers appear to be running out of energy' is the definition.
I can't tell whether this definition defines the answer.
'it is over where' is the wordplay.
'is over' says the letters should be written in reverse.
'where' becomes 'ring' (I can't explain this - if you can you should believe this answer much more).
'it' backwards is 'ti'.
'ti'+'ring'='TIRING'
Can you help me to learn more?
(Other definitions for tiring that I've seen before include "Causing weariness" , "Becoming sleepy" , "Boring" , "Wearisome" , "Laborious" .)