Opera lover's head is not unusual (6)
I believe the answer is:
normal
'not unusual' is the definition.
(I've seen this before)
'opera lover's head' is the wordplay.
'opera' becomes 'norma' (Vincenzo Bellini opera).
'head' says to take the initial letters.
The initial letter of 'lovers' is 'l'.
'norma'+'l'='NORMAL'
'is' acts as a link.
(Other definitions for normal that I've seen before include "Commonplace" , "Standard; at right angles" , "Usual, not extraordinary" , "Unexceptional" , "Conforming to what is usual" .)