Monster has registered an ally (6)
I believe the answer is:
friend
'ally' is the definition.
(I know that ally can be written as friend)
'monster has registered an' is the wordplay.
'monster' becomes 'fiend' (synonyms).
'has' indicates putting letters inside.
'registered an' becomes 'r' (this could be a standard abbreviation which I don't know about).
'fiend' going around 'r' is 'FRIEND'.
Can you help me to learn more?
(Other definitions for friend that I've seen before include "One in need is one indeed" , "Member of a religious community" , "Pal, amigo" , "China" , "Pal, mate" .)