The ache that's upset a wild cat (7)
I believe the answer is:
cheetah
'a wild cat' is the definition.
'cheetah' can be an answer for 'cat' (cheetah is a kind of cat). I'm unsure of the 'a wild' bit.
'the ache that's upset' is the wordplay.
'that's upset' indicates anagramming the letters.
'the'+'ache'='theache'
'theache' anagrammed gives 'CHEETAH'.
Can you help me to learn more?
(Other definitions for cheetah that I've seen before include "spotted in Africa" , "Swift, spotted feline" , "Long-legged spotted big cat" , "Swift-running cat" , "Large spotted wild cat, very fast" .)