Opera lover's head is not unusual (6)
I believe the answer is:
normal
'not unusual' is the definition.
(I've seen this in another clue)
'opera lover's head' is the wordplay.
'opera' becomes 'norma' (I've seen this in another clue).
'head' says to take the initial letters.
The first 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" .)