Pretty carriage seen in race (6)
I believe the answer is:
fairly
'pretty' is the definition.
(I know that pretty can be written as fairly)
'carriage seen in race' is the wordplay.
'carriage' becomes 'air' (both can mean someone's stance or posture).
'seen in' is an insertion indicator.
'race' becomes 'fly' (I've seen this in other clues).
'air' placed inside 'fly' is 'FAIRLY'.
(Other definitions for fairly that I've seen before include "Justly, without favouring one party" , "Evenhandedly" , "To a moderate degree" , "Equitably; passably" , "Rather" .)