Drive a prisoner to the end of his tether (5)
I believe the answer is:
power
'drive' is the definition.
(thesaurus)
'a prisoner to the end of his tether' is the wordplay.
'a prisoner' becomes 'pow' (prisoner of war).
'to the end of his tether' becomes 'er' (I can't explain this - if you can you should believe this answer much more).
'pow'+'er'='POWER'
Can you help me to learn more?
(Other definitions for power that I've seen before include "Strength or authority" , "Supremacy, force" , "Rate of doing work" , "Ability to act or physical might" , "military force" .)