How the loner may join (5)
I believe the answer is:
enrol
'join' is the definition.
(I've seen this before)
'how the loner' is the wordplay.
'how' indicates an anagram (I've seen this in other clues).
'loner' with letters rearranged gives 'ENROL'.
'may' is the link.
(Other definitions for enrol that I've seen before include "Sign on, being a loner" , "register for classes" , "Register (as member)" , "Loner inclined to join in" , "Record" .)