Fairly or smoothly (6)
I believe the answer is:
evenly
I believe this clue is a double definition.
'fairly' is the first definition.
(I've seen this before)
'smoothly' is the second definition.
(I've seen this before)
(Other definitions for evenly that I've seen before include "In level or uniform manner" , "In level way or manner" , "in regular fashion" , "constant rate" , "Material" .)