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 judge whether this defines the answer.
'it is over where' is the wordplay.
'is over' says the letters should be written backwards.
'where' becomes 'ring' (I can't explain this - if you can you should believe this answer much more).
'it' in reverse letter order 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" .)