Old ordered to rest strains (6)
I believe the answer is:
exerts
'strains' is the definition.
(I've seen this in another clue)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for exerts that I've seen before include "Brings something to bear forcefully" , "Forcefully applies (pressure)" , "Brings into use or to bear" , "Makes use of" , "Brings into use, say strength" .)