Rock singer’s run off, rocking (6)
I believe the answer is:
'rock' is the definition.
(gneiss is a kind of rock)
'singer's run off rocking' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'off' is a deletion indicator.
'rocking' is an anagram indicator.
'singers' with 'r' removed is 'singes'.
'singes' is an anagram of 'GNEISS'.
(Other definitions for gneiss that I've seen before include "Coarse-grained rock; sounds like agreeable" , "Metamorphic rock" .)