Crossword Genius

Criminal act, one perpetrated in prison? (6,3)

Ross

I believe the answer is:

inside job

'criminal act' is the definition.
(a crime committed with assistance from inside)

I cannot understand how the remainder of the clue works.

(Other definitions for inside job that I've seen before include "Crime by someone with special knowledge of the target" , "that an office worker experiences?" , "Crime of one hired before" , "breach of trust" , "Case" .)

I've seen this clue in The Times.
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