Lock partially securing letterbox (7)
I believe the answer is:
ringlet
'lock' is the definition.
(thesaurus)
'partially securing letterbox' is the wordplay.
'partially' indicates the answer is hidden within the clue.
'RINGLET' is hidden within 'securing letterbox'.
(Other definitions for ringlet that I've seen before include "Long curl of hair" , "Cluster of hair" , "Curling lock of hair" , "Curl, of an Irish dancer say" , "Little curl" .)