Spilt a rum on large painting (5)
I believe the answer is:
mural
'painting' is the definition.
(I know that mural is a type of painting)
'spilt a rum on large' is the wordplay.
'spilt' is an anagram indicator.
'on' means one lot of letters go next to another.
'large' becomes 'l' (eg in clothes sizes).
'a'+'rum'='arum'
'arum' with letters rearranged gives 'mura'.
'mura'+'l'='MURAL'
(Other definitions for mural that I've seen before include "Wall art" , "Wall decoration" , "Painting on a wall, fresco" , "Drawing on a wall" , "Wall design" .)