No rest, oddly, for singers (6)
I believe the answer is:
tenors
'for singers' is the definition.
(tenor is a singing voice)
'no rest oddly' is the wordplay.
'oddly' indicates anagramming the letters (letters in an odd order).
'no'+'rest'='norest'
'norest' is an anagram of 'TENORS'.
(Other definitions for tenors that I've seen before include "Male singers with high register" , "Opera singers" , "Male singers from Reston" , "Male singing voices" .)