Isn’t an awfully short time? It is (7)
I believe the answer is:
instant
'it is' is the definition.
The definition suggests a singular noun which matches the answer.
'isn't an awfully short time?' is the wordplay.
'awfully' is an anagram indicator.
'short time?' becomes 't' (abbreviation; 'short' suggests a short form).
'isnt'+'an'='isntan'
'isntan' with letters rearranged gives 'instan'.
'instan'+'t'='INSTANT'
(Other definitions for instant that I've seen before include "Happening immediately" , "Fast, flash" , "Jiffy" , "Twinkling of a eye" , "type of coffee" .)