It will have to be consulted before changing course (4)
I believe the answer is:
menu
'it will have to be consulted' is the definition.
The definition suggests a singular noun which matches the answer.
'before changing course' is the wordplay.
I cannot really understand how this works, but
'before' could be 'e' (I've seen this in other clues) and 'e' is found in the answer.
'course' could be 'n' and 'n' is present in the answer.
The remaining letters 'mu' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for menu that I've seen before include "List of items available in restaurant (4)" , "List of fare" , "List of items to choose from as in restaurant" , "Tariff" , "List of food items on offer" .)