Effort, toil (8)
I believe the answer is:
exertion
I believe this is a double definition.
'effort' is the first definition.
(synonyms)
'toil' is the second definition.
(exertion is a kind of toil)
(Other definitions for exertion that I've seen before include "Physical or mental effort" , "Tire oxen with heavy effort" , "Strenuous activity" , "Strain" , "labour" .)