Head guard smuggles note back for outsider (7)
I believe the answer is:
visitor
'outsider' is the definition.
(thesaurus)
'head guard smuggles note back' is the wordplay.
'head guard' becomes 'visor' (a visor is a guard for one's eyes. I am not sure about the 'head' bit.).
'smuggles' indicates putting letters inside.
'note' becomes 'ti' (musical note, as in do re mi so fa la ti do).
'back' says the letters should be written in reverse.
'ti' in reverse letter order is 'it'.
'visor' enclosing 'it' is 'VISITOR'.
'for' is the link.
Can you help me to learn more?
(Other definitions for visitor that I've seen before include "Seasonal migratory bird" , "Company" , "One coming calling" , "Guest - tourist" , "Caller, guest" .)