Nine legs running around school, weakening (9)
I believe the answer is:
lessening
'weakening' is the definition.
(lessening is a kind of weakening)
'nine legs running around school' is the wordplay.
'running' indicates anagramming the letters (the letters run around to different places).
'around' is an insertion indicator.
'school' becomes 'S'.
'nine'+'legs'='ninelegs'
'ninelegs' is an anagram of 'lesening'.
'lesening' going around 's' is 'LESSENING'.
(Other definitions for lessening that I've seen before include "Making fewer" , "Decreasing, diminishing" .)