Make false accusations when blue (7)
I believe the answer is:
asperse
'make false accusations' is the definition.
(as in casting aspersions)
'when blue' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'blue' becomes 'perse' (a shade of greyish blue).
'as'+'perse'='ASPERSE'
(Other definitions for asperse that I've seen before include "Slander or calumniate" , "Call names" , "Accuse falsely" , "See spar could slander oddly" , "Slander like Parsees" .)