Field boundary run exhausted energy (4,4)
I believe the answer is:
goal line
'field boundary' is the definition.
(eg boundary on a football field)
'run exhausted energy' is the wordplay.
'run' becomes 'go' (both can mean to function or work).
'exhausted' becomes 'all in' ('all in' can mean exhausted).
'energy' becomes 'e' (physics symbol as in E=mc2).
'go'+'allin'+'e'='GOAL-LINE'
(Other definitions for goal line that I've seen before include "Mark behind the keeper" , "End of a football pitch" , "A lion leg (anag.)" .)