Sings badly about English rock (6)
I believe the answer is:
gneiss
'rock' is the definition.
(I know that Gneiss is a type of Metamorphic rock)
'sings badly about english' is the wordplay.
'badly' indicates anagramming the letters.
'about' is an insertion indicator.
'english' becomes 'e' (abbreviation).
'sings' with letters rearranged gives 'gniss'.
'gniss' placed around 'e' is 'GNEISS'.
(Other definitions for gneiss that I've seen before include "Metamorphic rock" , "Coarse-grained rock; sounds like agreeable" .)