Affair involves head of Royal Mail perhaps (6)
I believe the answer is:
armour
'mail perhaps' is the definition.
(mail is a kind of armour)
'affair involves head of royal' is the wordplay.
'affair' becomes 'amour' (synonyms).
'involves' indicates putting letters inside.
'head of royal' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'amour' enclosing 'r' is 'ARMOUR'.
(Other definitions for armour that I've seen before include "Chain mail, e.g." , "defence" , "king caught in affair" , "Metal or chainmail suit" , "Greave, sallet and cuirass are pieces of this" .)