Measure ground, pocketing new client's key for access (8)
I believe the answer is:
username
'client's key for access' is the definition.
'measure ground pocketing new' is the wordplay.
'ground' indicates anagramming the letters (grind the letters ).
'pocketing' is an insertion indicator.
'new' becomes 'n'.
'measure' is an anagram of 'userame'.
'userame' going around 'n' is 'USERNAME'.
(Other definitions for username that I've seen before include "Log in identifier" , "Something needed for logging in" , "Computer log-in requirement" , "Electronic identity" , "One's identifier on a computer" .)