ERG: nowt affected Gavin's first blunder (3,5)

I believe the answer is:

get wrong

'blunder' is the definition.

'erg nowt affected gavin's first' is the wordplay.
'affected' indicates an anagram.
'gavin's first' becomes 'g' (1st letter of 'gavin').
'erg'+'nowt'='ergnowt'
'ergnowt' with letters rearranged gives 'getwron'.
'getwron'+'g'='GET WRONG'

I've seen this clue in The Independent.

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