A horse/sheep mix-up (7)
I believe the answer is:
anagram
'mix-up' is the definition.
(I've seen this before)
'a horsesheep' is the wordplay.
I cannot quite see how this works, but
'a' could be 'an' and 'an' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for anagram that I've seen before include "Eg "oscillation" of "colonialist"" , "Word rearranged from the letters of another" , "Word formed by rearranging another word" , "Scramble of letters (to make trestle, perhaps)" , "Mix-up" .)