Bill's comparatively astute (5)
I believe the answer is:
flyer
I believe this clue is a double definition.
'bill's' is the first definition.
(synonyms)
'comparatively astute' is the second definition.
(fly can mean astute or clever)
(Other definitions for flyer that I've seen before include "An aviator or fast mover" , "rook, for example" , "Pilot" , "Widely distributed leaflet" , "Promotional leaflet" .)