Is compelled to change a nag for a livelier horse (7)
I believe the answer is:
mustang
'livelier horse' is the definition.
'mustang' can be an answer for 'horse' (mustang is a kind of horse). I'm not certain of the 'livelier' bit.
'is compelled to change a nag' is the wordplay.
'is compelled to' becomes 'must' (I've seen this in another clue).
'change' indicates an anagram.
'nag' is an anagram of 'ang'.
'must'+'ang'='MUSTANG'
'for a' acts as a link.
Can you help me to learn more?
(Other definitions for mustang that I've seen before include "Hardy wild horse of the American western plains" , "SW US wild horse" , "Wild horse of the West" , "Feral American horse" , "one running wild" .)