Music genre behind Nadir (4,6)

I believe the answer is:

rock bottom

'nadir' is the definition.

'music genre behind' is the wordplay.
'music genre' becomes 'rock'.
'behind' becomes 'bottom' ('bottom' can be a synonym of 'behind').
'rock'+'bottom'='ROCK-BOTTOM'

(Other definitions for rock bottom that I've seen before include "such depths?" , "Record low" , "such a price is heavily reduced" , "Hard bed" , "Lowest possible level" .)

I've seen this clue in The Guardian.

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