Take complaint from mostly despicable character (3)
I believe the answer is:
'despicable character' is the definition.
(I've seen this in another clue)
'take complaint from mostly' is the wordplay.
'take complaint' becomes 'cu' (I can't justify this - if you can you should believe this answer much more).
'from' becomes 'fr' (abbreviation).
'mostly' means to remove the first letter.
'fr' with its first letter removed is 'r'.
Can you help me to learn more?
(Other definitions for cur that I've seen before include "Mongrel; rascal" , "Non-pedigree dog" , "Lowly dog" , "A worthless dog or contemptible person" , "Wretch" .)