Register as unemployed (4,2)
I believe the answer is:
sign on
'register as unemployed' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sign on that I've seen before include "claim unemployment benefit as well" , "Register as for social welfare payments" , "'Register, for job-seeker's allowance, say (4,2)'" , "Register, say as being unemployed" , "Register oneself as for employment assistance" .)