This field of activity isn't quite the real me (5)
I believe the answer is:
realm
'this field of activity' is the definition.
(I've seen this before)
'isn't quite the real me' is the wordplay.
'isn't quite' means to remove the last letter (not quite all of the word).
'real'+'me'='realme'
'realme' with its last letter taken away is 'REALM'.
(Other definitions for realm that I've seen before include "Kingdom; field of interest" , "Province" , "Area of interest/activity" , "Kingdom or sphere" , "field of study" .)