Predictable pattern in regular letters from Proust (3)
I believe the answer is:
rut
'predictable pattern' is the definition.
(a rut is a predictable pattern or routine)
'regular letters from proust' is the wordplay.
'regular letters from' means one should take alternating letters.
The alternate letters of 'proust' are 'RUT'.
'in' is the link.
(Other definitions for rut that I've seen before include "Deep track or dull routine" , "hazard in the road" , "humdrum activity" , "It can be frustrating" , "animal passion" .)