Discovered Boris intended missing an opening (7)
I believe the answer is:
orifice
'opening' is the definition.
(I know that opening can be written as orifice)
'discovered boris intended missing an' is the wordplay.
'discovered boris' becomes 'ori' (I can't justify this - if you can you should give a lot more credence to this answer).
'intended' becomes 'fiance' (who someone intends to marry).
'missing' indicates named letters should be taken away.
'fiance' with 'an' taken away is 'fice'.
'ori'+'fice'='ORIFICE'
Can you help me to learn more?
(Other definitions for orifice that I've seen before include "Bodily opening, mouth of cavity" , "Hole, opening" , "Finding opening" , "Vent" , "8 [OPENING]" .)