It is part of a musical review, still... (6)
I believe the answer is:
static
'still' is the definition.
(synonyms)
'it is part of a musical review' is the wordplay.
'is part of' is an insertion indicator (part of the inside).
'a musical' becomes 'cats' (Andrew Lloyd Webber musical).
'review' says the letters should be written in reverse.
'it' going inside 'cats' is 'citats'.
'citats' back-to-front is 'STATIC'.
(Other definitions for static that I've seen before include "fixed" , "Still - crackling on the radio" , "Still - radio interference" , "Still - type of electricity" , "problem with radio reception" .)