Loner resolved to join (5)
I believe the answer is:
enrol
'to join' is the definition.
(I've seen this in another clue)
'loner resolved' is the wordplay.
'resolved' is an anagram indicator.
'loner' with letters rearranged gives 'ENROL'.
(Other definitions for enrol that I've seen before include "Add to list of members" , "Become a member" , "Sign on, being a loner" , "Recruit" , "register for classes" .)