Allow smuggling one European stone (7)
I believe the answer is:
granite
'stone' is the definition.
(granite is a kind of stone)
'allow smuggling one european' is the wordplay.
'allow' becomes 'grant' (synonyms).
'smuggling' is an insertion indicator (I've seen this in other clues).
'one' becomes 'i' (roman numeral).
'european' becomes 'e' (abbreviation e.g. EU).
'grant' placed around 'i' is 'granit'.
'granit'+'e'='GRANITE'
(Other definitions for granite that I've seen before include "Tangier (anag.)" , "Showing great resolution" , "A very hard rock from Tangier" , "Crystalline rock" , "Hard stone from Tangier" .)