Short of time? (5,3)
I believe the answer is:
under age
'short of time?' is the definition.
The definition suggests a singular noun which matches the answer.
'short of time?' is the wordplay.
'short of' becomes 'under' (eg short of 10 means under 10).
'time?' becomes 'age' (an age is a period of time).
'under'+'age'='UNDER-AGE'
Can you help me to learn more?
(Other definitions for under age that I've seen before include "Too young" , "Immature" , "less than 18?" , "Like a minor" .)