Slogan left in clay pot (3,4)
I believe the answer is:
tag line
'slogan' is the definition.
(I've seen this before)
'left in clay pot' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'clay pot' becomes 'tagine' (I can't explain this - if you can you should believe this answer much more).
'l' put into 'tagine' is 'TAG LINE'.
Can you help me to learn more?
(Other definitions for tag line that I've seen before include "Something memorable one might say" , "Catchphrase (mainly US)" , "Slogan" , "End of joke" .)