Go up? Not when this is the case! (4,3)
I believe the answer is:
lift off
'the case' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see a link between them that I don't see?
'go up? not when' is the wordplay.
'go up?' becomes 'lift' (going up is a kind of lifting).
'not' becomes 'o' (I've seen this in other clues).
'when' becomes 'ff' (I can't justify this - if you can you should believe this answer much more).
'lift'+'o'+'ff'='LIFT-OFF'
'this is' acts as a link.
Can you help me to learn more?
(Other definitions for lift off that I've seen before include "What astronauts have to" , "launch of spacecraft?" , "Leave ground" , "Why stairs are used" , "make them use the stairs?" .)