Used by a bowman leading Don Quixote's horse (5)
I believe the answer is:
rosin
'don quixote's horse' is the definition.
I can't tell whether this definition defines the answer.
'used by a bowman leading' is the wordplay.
'used by' says to put letters next to each other.
'a bowman' becomes 'ros' (I can't justify this - if you can you should believe this answer much more).
'leading' becomes 'in' (in government).
'ros' put next to 'in' is 'ROSIN'.
Can you help me to learn more?
(Other definitions for rosin that I've seen before include "No sir; it is rubbed on violin bows" , "Sticky stuff rubbed on violin strings" , "It's used on a violin bow" , "Colophony" , "What gives bite to strings" .)