Strenuous exertion (6)
I believe the answer is:
effort
'strenuous exertion' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for effort that I've seen before include "For eft (anag.)" , "Strain" , "Earnest attempt" , "Attempt, energy exerted" , "Attempt, endeavour" .)