Effect blue change twice (6)
I believe the answer is:
double
'twice' is the definition.
(synonyms)
'effect blue change' is the wordplay.
'effect' becomes 'do' (synonyms).
'change' is an anagram indicator.
'blue' anagrammed gives 'uble'.
'do'+'uble'='DOUBLE'
(Other definitions for double that I've seen before include "Designed for two users" , "With two equal parts" , "Dead ringer" , "Offence of leaving one's car where it blocks another" , "Ringer - large drink" .)