Sign in to a computer (3,2)
I believe the answer is:
log on
'sign in to a computer' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for log on that I've seen before include "Prepare to surf, perhaps?" , "what the lumberjack's lorry may have?" , "Access the Internet" , "Gain computer access" , "Sign in to a computer" .)