Sign up strange loner (5)
I believe the answer is:
enrol
'sign up' is the definition.
(I've seen this before)
'strange loner' is the wordplay.
'strange' indicates anagramming the letters.
'loner' with letters rearranged gives 'ENROL'.
(Other definitions for enrol that I've seen before include "Insert in register" , "register for classes" , "Become a member" , "Bring into service" , "Loner inclined to join in" .)