Marines in future mount ritual (6)
I believe the answer is:
formal
'ritual' is the definition.
(thesaurus)
'marines in future mount' is the wordplay.
'marines' becomes 'RM' (abbreviation for 'Royal Marines').
'in' is an insertion indicator.
'future mount' becomes 'foal' (I am not sure about this - if you are sure you should believe this answer much more).
'rm' going into 'foal' is 'FORMAL'.
Can you help me to learn more?
(Other definitions for formal that I've seen before include "Officially recognised" , "Stiff - proper" , "Cermonial, solemn" , "conventionally correct" , "Like court dress, perhaps" .)