Understudy is on first, and going to pieces! (5,2,3)

I believe the answer is:

stand in for

'understudy' is the definition.
(thesaurus)

'on first and going to pieces' is the wordplay.
'going to pieces' indicates anagramming the letters (going to pieces can mean breaking down).
'on'+'first'+'and'='onfirstand'
'onfirstand' anagrammed gives 'STAND IN FOR'.

'is' acts as a link.

I've seen this clue in The Independent.

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