Work on extension, usually very reliable, erratic initially (6)
I believe the answer is:
oeuvre
'work' is the definition.
(I've seen this before)
'on extension usually very reliable erratic initially' is the wordplay.
'initially' says to take the initial letters.
The initial letters of 'on extension usually very reliable erratic' is 'OEUVRE'.
(Other definitions for oeuvre that I've seen before include "Body of artistic or literary work" , "All the works of a particular composer" , "Collected works" , "Sum of all works" , "Body of work of an artist (6)" .)