Smoothly, uniformly (6)
I believe the answer is:
evenly
This clue is a double definition.
'smoothly' is the first definition.
(I've seen this before)
'uniformly' is the second definition.
(I've seen this before)
(Other definitions for evenly that I've seen before include "On an equal basis" , "In measured and level way" , "not getting ruffled" , "Smoothly or fairly" , "In level or uniform manner" .)