I'll leave fast food firm, having painted wall (6)
I believe the answer is:
fresco
'painted wall' is the definition.
(a fresco is a painting done on a wall)
'i'll leave fast food firm' is the wordplay.
I cannot quite see how this works, but
'food' could be 'e' (E is an example) and 'e' is found in the answer.
'firm' could be 'co' (a firm is a company) and 'co' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'having' is the link.
(Other definitions for fresco that I've seen before include "Painting using watercolours on plaster" , "Picture painted on wall or ceiling before plaster is dry" , "Work of art" , "Mode of wall painting" , "Mural painting" .)