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' is an insertion indicator.
'note' becomes 'ti' (musical note, as in do re mi so fa la ti do).
'back' is a reversal indicator.
'ti' written backwards gives 'it'.
'visor' placed around 'it' is 'VISITOR'.
'for' acts as a 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" .)