End of butter and milk source guide (6)
I believe the answer is:
rudder
'guide' is the definition.
(means of guiding a boat)
'end of butter and milk source' is the wordplay.
'end of' suggests the final letters.
'and' says to put letters next to each other.
'milk source' becomes 'udder' (I've seen this in another clue).
The final letter of 'butter' is 'r'.
'r'+'udder'='RUDDER'
(Other definitions for rudder that I've seen before include "Ship's steering plate" , "Ship's steering device" , "Steering device on boat" , "Control surface on plane's tail" , "Boat's steering apparatus" .)