Brief passage: "furry mice, mule and mare skinned outside school" (15)
I believe the answer is:
extracurricular
'brief' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I don't see?
'passage furry mice mule and mare skinned outside school' is the wordplay.
I cannot really see how this works, but
'passage' could be 'cul' (cul is a kind of passage) and 'cul' is present in the answer.
'outside' could be 'extra' (similar in meaning) and 'extra' is found in the answer.
This explanation may well be incorrect...
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" .)