Moderately (6)
(Other definitions for fairly that I've seen before include "pretty" , "Evenhandedly" , "Equitably; passably" , "To a moderate degree" , "Rather" .)
'moderately' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for somewhat that I've seen before include "Rather, a little" , "To a certain extent" , "To an extent, a little" , "Moderately" , "up to a point" .)