Allow to undergo (6)
I believe the answer is:
suffer
This clue is a double definition.
'allow' is the first definition.
(suffering is a kind of allowing)
'undergo' is the second definition.
(suffering is a kind of undergoing)
'to' is the link.
(Other definitions for suffer that I've seen before include "Experience (something bad)" , "Endure; hurt" , "Be affected by (illness)" , "Undergo pain or grief" , "Endure pain or discomfort" .)