Sign name to gain access to system (3,2)
I believe the answer is:
log on
'gain access to system' is the definition.
'sign name' is the wordplay.
'sign' becomes 'logo' (logo is a kind of sign).
'name' becomes 'n' (common abbreviation e.g. on forms).
'logo'+'n'='LOG ON'
'to' is the link.
(Other definitions for log on that I've seen before include "Gain computer access" , "Sign in to a computer" , "what the lumberjack's lorry may have?" , "access network" , "Enter online program" .)