Lodger loses article at the frontier (6)
I believe the answer is:
border
'the frontier' is the definition.
(I've seen this before)
'lodger loses article' is the wordplay.
'lodger' becomes 'boarder' (synonyms).
'loses' suggests deleting specific letters.
'article' becomes 'a' (the indefinite article in English).
'boarder' with 'a' removed is 'BORDER'.
'at' acts as a link.
(Other definitions for border that I've seen before include "bed?" , "skirt" , "Margin along the edge" , "Line that indicates a boundry" , "Rim" .)