Recalled old college servant painting on wall (6)
I believe the answer is:
fresco
'painting on wall' is the definition.
(frescos are usually painted on walls)
'recalled old college servant' is the wordplay.
'recalled' says the letters should be written in reverse.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'college' becomes 'c'.
'servant' becomes 'serf' (serf is a kind of servant).
'o'+'c'+'serf'='ocserf'
'ocserf' written backwards gives 'FRESCO'.
(Other definitions for fresco that I've seen before include "Wall-painting" , "Picture painted on wall or ceiling before plaster is dry" , "Painting using watercolours on plaster" , "Work of art" , "Method of painting on walls" .)