Stipulation (7)
I believe the answer is:
proviso
'stipulation' is the definition.
(I know that proviso is a type of stipulation)
This is all the clue.
(Other definitions for proviso that I've seen before include "Condition of stipulation of an agreement" , "Condition added to an agreement" , "Something insisted upon as part of an agreement" , "Requirement" , "Rider" .)