Activity that makes oxen tire (8)
I believe the answer is:
exertion
'activity' is the definition.
(exertion is a kind of activity)
'makes oxen tire' is the wordplay.
'makes' is an anagram indicator.
'oxen'+'tire'='oxentire'
'oxentire' with letters rearranged gives 'EXERTION'.
'that' acts as a link.
(Other definitions for exertion that I've seen before include "labour" , "Tire oxen with heavy effort" , "Strenuous activity" , "Strain" , "Use of physical energy" .)