Sign loner up (5)
I believe the answer is:
enrol
'sign' is the definition.
The definition and answer can be both to do with social activities as well as being verbs in their base form.
Perhaps there's an association between them I don't understand?
'loner up' is the wordplay.
'up' indicates anagramming the letters.
'loner' with letters rearranged gives 'ENROL'.
Can you help me to learn more?
(Other definitions for enrol that I've seen before include "Join up, sign on" , "Join as member" , "Sign on, being a loner" , "Register (as student)" , "Record" .)