Singer is bighead and idiot (4)
I believe the answer is:
bass
'singer' is the definition.
(I know that bass is a type of singer)
'bighead and idiot' is the wordplay.
'bighead' becomes 'b' (the head letter of 'big').
'and' means one lot of letters go next to another.
'idiot' becomes 'ass' (I've seen this in another clue).
'b'+'ass'='BASS'
'is' is the link.
(Other definitions for bass that I've seen before include "Fish - low voice" , "Performer" , "Booming percussion instrument" , "deep in the sound" , "Low singing voice" .)