A groundless scare - from a faulty clock? (5,5)
I believe the answer is:
false alarm
'a groundless scare' is the definition.
The definition suggests a singular noun which matches the answer.
'a faulty clock?' is the wordplay.
'a faulty' becomes 'false' (I can't justify this - if you can you should give a lot more credence to this answer).
'clock?' becomes 'alarm' (alarm is a kind of clock).
'false'+'alarm'='FALSE ALARM'
'from' acts as a link.
Can you help me to learn more?
(Other definitions for false alarm that I've seen before include "unfounded scare?" , "Warning proving unnecessary" , "Panic over" .)