Exhausted boxer, say, tried to change (3-5)
I believe the answer is:
'exhausted' is the definition.
(I know that exhausted can be written as dog-tired)
'boxer say tried to change' is the wordplay.
'boxer say' becomes 'dog' (boxer is a kind of dog).
'to change' indicates an anagram.
'tried' with letters rearranged gives 'tired'.
(Other definitions for dog-tired that I've seen before include "Worn out" , "Extremely weary" , "Exhausted" .)