Good thing to do when dog tired (3)
I believe the answer is:
sit
'do when dog tired' is the definition.
I can't tell whether this definition defines the answer.
'good thing' is the wordplay.
I cannot really see how this works, but
'thing' could be 'it' and 'it' is found within the answer.
A single letter 's' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for sit that I've seen before include "Brood" , "Take weight off feet" , "Take a pew, an exam" , "Order dog to behave" , "Model" .)