Gathered forces for painting on a wall (6)
I believe the answer is:
fresco
'painting on a wall' is the definition.
'fresco' can be an answer for 'wall' (a fresco is a painting done on a wall). I'm not certain of the 'painting on a' bit.
'gathered forces' is the wordplay.
'gathered' indicates an anagram (the letters are gathered into a new order).
'forces' anagrammed gives 'FRESCO'.
'for' is the link.
Can you help me to learn more?
(Other definitions for fresco that I've seen before include "Mural painted on a wall" , "Painting done in wet plaster" , "Mural painted on wet plaster" , "Picture painted on wet plaster" , "Artwork" .)