Usual? Not on a butterfly (5)
I believe the answer is:
comma
'butterfly' is the definition.
(comma is a kind of butterfly)
'usual? not on a' is the wordplay.
'usual?' becomes 'common' ('common' can be similar in meaning to 'usual').
'not' suggests deleting specific letters.
'common' with 'on' taken away is 'comm'.
'comm'+'a'='COMMA'
(Other definitions for comma that I've seen before include "Mark for smallest division of a sentence" , "Common punctuation park" , "its use may be divisive" , "repeatedly found in lists" , "breathtaking marking?" .)