Raiders composing start of letter (4,3)
I believe the answer is:
dear sir
'start of letter' is the definition.
I know nothing about this answer so I can't judge whether this works.
'raiders composing' is the wordplay.
'composing' indicates an anagram.
'raiders' anagrammed gives 'DEAR SIR'.
Can you help me to learn more?
(Other definitions for dear sir that I've seen before include "Conventional opening address to male recipient of letter" , "Opening words of formal letter" , "Conventional letter opening to man" , "Form of address" , "Traditional intro" .)