In books, Peter Pan gets acting work (8)
I believe the answer is:
'work' is the definition.
(operetta is a type of musical work)
'in books peter pan gets acting' is the wordplay.
'in' means one lot of letters goes inside another.
'books' becomes 'ot' (abbreviation for Old Testament).
'pan' indicates an anagram (letters must be panned or attacked).
'gets' is a charade indicator (letters next to each other).
'acting' becomes 'a' (abbreviation, eg in job titles).
'peter' is an anagram of 'peret'.
'ot' enclosing 'peret' is 'operett'.
(Other definitions for operetta that I've seen before include "Light opera" , "G&S production, perhaps" , "Light classical musical comedy-drama" , "The Merry Widow, for example" , "Eg, Orpheus in the Underworld" .)