One guide of a good many (5)
I believe the answer is:
pilot
'one guide' is the definition.
(I've seen this before)
'a good many' is the wordplay.
'a good' becomes 'pi' (I've seen this in other clues).
'many' becomes 'lot' (I've seen this before).
'pi'+'lot'='PILOT'
'of' is the 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" .)