Bugs Bunny's opening song impressing cutie, occasionally (8)
I believe the answer is:
bacteria
'bugs' is the definition.
(I've seen this before)
'bunny's opening song impressing cutie occasionally' is the wordplay.
'opening' says to take the initial letters.
'song' becomes 'aria' (aria is a kind of song).
'impressing' is an insertion indicator (impress historically meant to conscript).
'occasionally' indicates alternate letters.
The initial letter of 'bunny' is 'b'.
The alternate letters of 'cutie' are 'cte'.
'aria' placed around 'cte' is 'acteria'.
'b'+'acteria'='BACTERIA'
(Other definitions for bacteria that I've seen before include "Micro-organisms, often pathogens" , "Single-celled organisms" , "Single-cell organism" , "Germs or bugs" , "Microscopic organisms, may cause disease" .)