Not allowed on tin of tobacco (5)
I believe the answer is:
snout
'tobacco' is the definition.
(I've seen this before)
'not allowed on tin' is the wordplay.
'not allowed' becomes 'out' (I've seen this before).
'on' means one lot of letters go next to another.
'tin' becomes 'sn' (Sn is the chemical symbol for tin).
'out' put after 'sn' is 'SNOUT'.
'of' acts as a link.
(Other definitions for snout that I've seen before include "Nose, tobacco (colloq.)" , "Schnozzle" , "Nose; tobacco (slang)" , "nose of pig" , "Nose and mouth of a mammal" .)