Stage at which it should be smooth (5)
I believe the answer is:
level
This could be a double definition.
'stage at which' is the first definition.
'level' can be an answer for 'stage' (synonyms). I'm unsure of the remainder of the definition.
'smooth' is the second definition.
(I've seen this before)
'it should be' acts as a link.
Can you help me to learn more?
(Other definitions for level that I've seen before include "Flatten" , "Device ot establish a flat surface" , "Palindromic storey" , "Even; a plane" , "Tier; calm and steady" .)