Open with a single innuendo (8)
I believe the answer is:
overtone
'innuendo' is the definition.
(I've seen this before)
'open with a single' is the wordplay.
'open' becomes 'overt' ('overt' is associated in meaning with 'open'**).
'with' means one lot of letters go next to another.
'a single' becomes 'one' ('one' can be a synonym of 'single').
'overt'+'one'='OVERTONE'
(Other definitions for overtone that I've seen before include "Association" , "Subtle extra meaning" , "Hint" , "hidden meaning" , "Subtle quality" .)