Low way of working on an inside job (3)
I believe the answer is:
Here is my best explanation:
'low' is the definition.
('moo' can be a synonym of 'low')
'way of working on an inside job' is the subsidiary indication.
'way of working' becomes 'mo' (I've seen this in other clues).
'on' is a charade indicator (letters next to each other).
'an inside' means to look at the middle letters.
The middle letters of 'job' are 'o'.
(Other definitions for moo that I've seen before include "Cow's call", "Cartoon sound of a cow", "Guernsey noise?", "Cow's low", "Irritating woman", "Sound of cow", "Bovine low", "unpopular woman", "Low (of cattle)" and "Cow's sound".)