Brief passage: "furry mice, mule and mare skinned outside school" (15)
I believe the answer is:
extracurricular
'brief' is the definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I can't see?
'passage furry mice mule and mare skinned outside school' is the wordplay.
I cannot really understand how this works, but
'passage' could be 'cul' (cul is a kind of passage) and 'cul' is located in the answer.
'outside' could be 'extra' (similar in meaning) and 'extra' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for extracurricular that I've seen before include "Outside the normal school subjects" , "Outside the usual syllabus" .)