Run in unaccompanied, turning sign on (5)
I believe the answer is:
enrol
'sign on' is the definition.
(I've seen this before)
'run in unaccompanied turning' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'in' is an insertion indicator.
'unaccompanied' becomes 'lone' (I've seen this before).
'turning' shows that the letters should be reversed in order.
'r' put within 'lone' is 'lorne'.
'lorne' written backwards gives 'ENROL'.
(Other definitions for enrol that I've seen before include "Bring into service" , "Insert in register" , "Add to list of members" , "Sign on, being a loner" , "Register officially" .)