Monster gets registered as a chum (6)
I believe the answer is:
friend
'a chum' is the definition.
(I've seen this in another clue)
'monster gets registered as' is the wordplay.
'monster' becomes 'fiend' (synonyms).
'gets' indicates putting letters inside (gets can mean captures or absorbs).
'registered as' becomes 'r' (abbreviation, as in registered trademark. I am not sure about the 'as' bit.).
'fiend' enclosing 'r' is 'FRIEND'.
Can you help me to learn more?
(Other definitions for friend that I've seen before include "allowed to pass by the guard" , "Member of religious society" , "Pal or buddy" , "Pal, mate" , "One you know well and regard with affection" .)