Cat saves energy: one controlling dog (5)
I believe the answer is:
leash
'controlling dog' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'cat saves energy one' is the wordplay.
'cat' becomes 'lash' (I've seen this in another clue).
'saves' is an insertion indicator.
'energy one' becomes 'e' (I can't explain this - if you can you should give a lot more credence to this answer).
'lash' enclosing 'e' is 'LEASH'.
Can you help me to learn more?
(Other definitions for leash that I've seen before include "Rope or chain restraint" , "Strap for holding a dog while walking" , "Rope to control dog" , "Dog lead" , "Strip for leading, a dog say" .)