Change of blue soil (6)
I believe the answer is:
befoul
'soil' is the definition.
(I've seen this in another clue)
'change of blue' is the wordplay.
'change' indicates an anagram.
'of'+'blue'='ofblue'
'ofblue' is an anagram of 'BEFOUL'.
(Other definitions for befoul that I've seen before include "Soil, defile" , "Make dirty" , "Pollute" .)