Singers in nest, or bats? (6)
I believe the answer is:
tenors
'singers' is the definition.
(tenor is a singing voice)
'nest or bats?' is the wordplay.
'bats?' is an anagram indicator (slang for mad).
'nest'+'or'='nestor'
'nestor' is an anagram of 'TENORS'.
'in' acts as a link.
(Other definitions for tenors that I've seen before include "Opera singers" , "Male singers from Reston" , "Male singers with high register" , "Male singing voices" .)