Registers terrible loners (6)
I believe the answer is:
enrols
'registers' is the definition.
(enrolling is a kind of registering)
'terrible loners' is the wordplay.
'terrible' is an anagram indicator.
'loners' anagrammed gives 'ENROLS'.
(Other definitions for enrols that I've seen before include "recruits" , "Enlists" , "Joins up with loners" , "Registers name" , "Enters on a list" .)