Working quietly to block debatable space mission (8)
I believe the answer is:
moonshot
'space mission' is the definition.
The definition suggests a singular noun which matches the answer.
'working quietly to block debatable' is the wordplay.
'working' becomes 'on'.
'quietly' becomes 'sh'.
'to block' is an insertion indicator (block can mean to fill a hole in something).
'debatable' becomes 'moot' ('moot' can be a synonym of 'debatable').
'on'+'sh'='onsh'
'onsh' going inside 'moot' is 'MOONSHOT'.
Can you help me to learn more?
(Other definitions for moonshot that I've seen before include "Launching of a lunar spacecraft" , "Ambitious launch" , "Space flight" , "Rocket launch to our nearest neighbour" , "Launch of spacecraft to Earth's near neighbour" .)