It's useful to those in a tearing hurry (11)
I believe the answer is:
perforation
'tearing hurry' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'it's useful to those in a' is the wordplay.
I cannot really see how this works, but
'to' could be 'for' and 'for' is located in the answer.
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is found within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for perforation that I've seen before include "holes?" , "Boring" .)