Smudge most of blue and red to start with (4)
I believe the answer is:
blur
'smudge' is the definition.
('blur' can be a synonym of 'smudge')
'most of blue and red to start with' is the wordplay.
'most of' means to remove the last letter.
'and' says to put letters next to each other.
'to start with' indicates taking the first letters.
The initial letter of 'red' is 'r'.
'blue' with its final letter taken away is 'blu'.
'blu'+'r'='BLUR'
(Other definitions for blur that I've seen before include "Haze - obscure" , "Become unclear" , "Befog" , "Unclear image, of a pop band" , "Indistinct appearance like smear" .)