Second-rate idiot as a singer (4)
I believe the answer is:
bass
'a singer' is the definition.
(bass is a kind of singer)
'second-rate idiot' is the wordplay.
'second-rate' becomes 'b' (second grade after A**).
'idiot' becomes 'ass' (I've seen this before).
'b'+'ass'='BASS'
'as' is the link.
(Other definitions for bass that I've seen before include "Deep-toned" , "Sea perch - male voice" , "Low singing voice" , "Scottish rock" , "Very low - perch" .)