Fool is up for a form of civil disobedience
I believe the answer is:
sit-in
'a form of civil disobedience' is the definition.
(I've seen this before)
'fool is up' is the wordplay.
'fool' becomes 'nit' (I've seen this before).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'nit'+'is'='nitis'
'nitis' reversed gives 'SIT-IN'.
'for' is the link.
(Other definitions for sit-in that I've seen before include "Occupation of building as form of protest" , "Form of civil disobedience" , "Act of civil disobedience" , "Protest occupying building" , "Occupation demo" .)