Drivel from one crying outside back of school (7)
I believe the answer is:
slobber
'drivel' is the definition.
(I know that drivel can be written as slobber)
'one crying outside back of school' is the wordplay.
'one crying' becomes 'sobber' (someone sobbing).
'outside' indicates putting letters inside.
'back of' indicates one should take the final letters.
The final letter of 'school' is 'l'.
'sobber' enclosing 'l' is 'SLOBBER'.
'from' acts as a link.
(Other definitions for slobber that I've seen before include "Behave with excessive affection, dribble" , "Slaver" , "Drool disgustingly" , "Dribble copiously" , "Salivate excessively" .)