One guide of a good many (5)
I believe the answer is:
pilot
'one guide' is the definition.
(pilot can mean a navigator)
'good many' is the wordplay.
'good' becomes 'pi' (old-fashioned term for pious).
'many' becomes 'lot' (a lot of something).
'pi'+'lot'='PILOT'
'of a' acts as a link.
(Other definitions for pilot that I've seen before include "Name of Mr Rochester's dog" , "This person can be found in a plane or a boat" , "Steer (a ship or plane)" , "Guide, steer" , "Driver of aircraft" .)