Disposition of a vanguard force at the Queen’s disposal (5-3-6)
I believe the answer is:
Here is my best explanation:
'the queen's disposal' is the definition.
The definition suggests a singular noun which matches the answer.
'disposition of a vanguard force' is the wordplay.
'disposition of' indicates an anagram (I've seen 'disposition' mean this).
'avanguardforce' with letters rearranged gives 'GRACE-AND-FAVOUR'.
'at' acts as a link.