Belittle act, blue (2,4)
I believe the answer is:
do down
'belittle' is the definition.
'act blue' is the wordplay.
'act' becomes 'do' (I've seen this in other clues**).
'blue' becomes 'down' (I've seen this before).
'do'+'down'='DO DOWN'
(Other definitions for do down that I've seen before include "Criticise" , "Cheat" , "Informally, get the better of or swindle" .)