Crossword Genius

Six-footer humming middle of sestet in key's first error (5,3)

Ross

I believe the answer is:

stink bug

'six-footer humming' is the definition.
(humming can mean stinky or smelly)

'middle of sestet in key's first error' is the wordplay.
'middle of' indicates the central letters.
'key's first' becomes 'k' (1st letter of 'key').
'error' becomes 'bug' (computer error).
The middle of 'sestet' is 'st'.
'st'+'in'+'k'+'bug'='STINK BUG'

I've seen this clue in The Guardian.
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