Disinclination to exertion (5)
I believe the answer is:
sloth
'disinclination to exertion' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sloth that I've seen before include "reluctance to work" , "One of the seven deadly sins" , "Deadly sin - upside-down mammal" , "A deadly sin, being lazy" , "one of a deadly collection" .)