High time? It could be! (5,4)
I believe the answer is:
happy hour
'it could be' is the definition.
The definition suggests a singular noun which matches the answer.
'high time?' is the wordplay.
'high' becomes 'happy' (I am not sure about this - if you are sure you should believe this answer much more).
'time?' becomes 'hour' (unit of time).
'happy'+'hour'='HAPPY HOUR'
Can you help me to learn more?
(Other definitions for happy hour that I've seen before include "Cheap drinking time" , "Cheap time to get merry!" , "pub's promotion" , "Period of reduced prices in public bar" , "glasses are now reduced" .)