Paint wall at rear in fancy yellow (7)
I believe the answer is:
gutless
'yellow' is the definition.
(I've seen this before)
'paint wall at rear in fancy' is the wordplay.
'at rear' says to take the final letters (the letter at the rear or back of the word).
'in' indicates putting letters inside.
'fancy' becomes 'guess' (I've seen this before).
The last letters of 'paint wall' are 'tl'.
'tl' inserted into 'guess' is 'GUTLESS'.
(Other definitions for gutless that I've seen before include "Showing no evidence of bottle" , "Chicken-livered" , "lacking a beer belly?" , "chicken?" , "Lacking courage and determination" .)